Fixes #672

* Downcase signed_headers string before building the signed string

The HTTP Signatures draft does not mandate the “headers” field to be downcased,
but mandates the header field names to be downcased in the signed string, which
means that prior to this patch, Mastodon could fail to process signatures from
some compliant clients. It also means that it would not actually check the
Digest of non-compliant clients that wouldn't use a lowercased Digest field
name.

Thankfully, I don't know of any such client.

* Revert "Remove dead code (#8919)"

This reverts commit a00ce8c92c.

* Restore time window checking, change it to 12 hours

By checking the Date header, we can prevent replaying old vulnerable
signatures. The focus is to prevent replaying old vulnerable requests
from software that has been fixed in the meantime, so a somewhat long
window should be fine and accounts for timezone misconfiguration.

* Escape users' URLs when formatting them

Fixes possible HTML injection

* Escape all string interpolations in Formatter class

Slightly improve performance by reducing class allocations
from repeated Formatter#encode calls

* Fix code style issues

Follow-up to #8766

Regression from #8942

Regression from #8957

* Downcase signed_headers string before building the signed string

The HTTP Signatures draft does not mandate the “headers” field to be downcased,
but mandates the header field names to be downcased in the signed string, which
means that prior to this patch, Mastodon could fail to process signatures from
some compliant clients. It also means that it would not actually check the
Digest of non-compliant clients that wouldn't use a lowercased Digest field
name.

Thankfully, I don't know of any such client.

* Revert "Remove dead code (#8919)"

This reverts commit a00ce8c92c.

* Restore time window checking, change it to 12 hours

By checking the Date header, we can prevent replaying old vulnerable
signatures. The focus is to prevent replaying old vulnerable requests
from software that has been fixed in the meantime, so a somewhat long
window should be fine and accounts for timezone misconfiguration.

* Escape users' URLs when formatting them

Fixes possible HTML injection

* Escape all string interpolations in Formatter class

Slightly improve performance by reducing class allocations
from repeated Formatter#encode calls

* Fix code style issues

Merge upstream changes

* Set CSP rules in RoR's configuration

* Override CSP setting in the embed controller to allow frames

Bumps [doorkeeper](https://github.com/doorkeeper-gem/doorkeeper) from 5.0.0 to 5.0.1.
- [Release notes](https://github.com/doorkeeper-gem/doorkeeper/releases)
- [Changelog](https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md)
- [Commits](https://github.com/doorkeeper-gem/doorkeeper/compare/v5.0.0...v5.0.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

* Add check for missing tag param in streaming API

Fixes error:

```
TypeError: Cannot read property 'toLowerCase' of undefined
at app.get (.../streaming/index.js:493:50)
```

* Fix code style issues

Regression from #8951

* Do not push DMs into the home feed

* Show DMs column after sending a DM, if DMs column is not already shown

Mention and emoji code may perform network calls, but does not need
to do that inside the database transaction. This may improve availability
of database connections when using pgBouncer in transaction mode.

Bumps [better_errors](https://github.com/BetterErrors/better_errors) from 2.4.0 to 2.5.0.
- [Release notes](https://github.com/BetterErrors/better_errors/releases)
- [Changelog](https://github.com/BetterErrors/better_errors/blob/master/CHANGELOG.md)
- [Commits](https://github.com/BetterErrors/better_errors/compare/v2.4.0...v2.5.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [bullet](https://github.com/flyerhzm/bullet) from 5.7.5 to 5.7.6.
- [Release notes](https://github.com/flyerhzm/bullet/releases)
- [Changelog](https://github.com/flyerhzm/bullet/blob/master/CHANGELOG.md)
- [Commits](https://github.com/flyerhzm/bullet/compare/5.7.5...5.7.6)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [scss_lint](https://github.com/brigade/scss-lint) from 0.57.0 to 0.57.1.
- [Release notes](https://github.com/brigade/scss-lint/releases)
- [Changelog](https://github.com/brigade/scss-lint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/brigade/scss-lint/compare/v0.57.0...v0.57.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Fix #8685

Conflicts:
	app/javascript/packs/public.js

Changes made to app/javascript/packs/publics.js were applied to
app/javascript/core/settings.js

Bumps [capistrano-rails](https://github.com/capistrano/rails) from 1.3.1 to 1.4.0.
- [Release notes](https://github.com/capistrano/rails/releases)
- [Changelog](https://github.com/capistrano/rails/blob/master/CHANGELOG.md)
- [Commits](https://github.com/capistrano/rails/compare/v1.3.1...v1.4.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [dotenv-rails](https://github.com/bkeepers/dotenv) from 2.2.2 to 2.5.0.
- [Release notes](https://github.com/bkeepers/dotenv/releases)
- [Changelog](https://github.com/bkeepers/dotenv/blob/master/Changelog.md)
- [Commits](https://github.com/bkeepers/dotenv/compare/v2.2.2...v2.5.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Merge upstream changes

Bumps [faker](https://github.com/stympy/faker) from 1.8.7 to 1.9.1.
- [Release notes](https://github.com/stympy/faker/releases)
- [Changelog](https://github.com/stympy/faker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/stympy/faker/compare/v1.8.7...v1.9.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [memory_profiler](https://github.com/SamSaffron/memory_profiler) from 0.9.11 to 0.9.12.
- [Release notes](https://github.com/SamSaffron/memory_profiler/releases)
- [Changelog](https://github.com/SamSaffron/memory_profiler/blob/master/CHANGELOG.md)
- [Commits](https://github.com/SamSaffron/memory_profiler/compare/v0.9.11...v0.9.12)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Merge upstream changes

* add ffmpeg initializer

* use different expression to check for environment var

Bumps [capybara](https://github.com/teamcapybara/capybara) from 3.8.2 to 3.9.0.
- [Release notes](https://github.com/teamcapybara/capybara/releases)
- [Changelog](https://github.com/teamcapybara/capybara/blob/master/History.md)
- [Commits](https://github.com/teamcapybara/capybara/compare/3.8.2...3.9.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [tty-prompt](https://github.com/piotrmurach/tty-prompt) from 0.17.0 to 0.17.1.
- [Release notes](https://github.com/piotrmurach/tty-prompt/releases)
- [Changelog](https://github.com/piotrmurach/tty-prompt/blob/master/CHANGELOG.md)
- [Commits](https://github.com/piotrmurach/tty-prompt/compare/v0.17.0...v0.17.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>