Fix #11969

Regression from #8657

* Add HTTP signature requirement for served ActivityPub resources

* Change `SECURE_MODE` to `AUTHORIZED_FETCH`

* Add 'Signature' to 'Vary' header and improve code style

* Improve code style by adding `public_fetch_mode?` method

* Remove skip_session! as it is not supported in Rails 5

* Minor cleanup in StreamEntriesController

* Remove redundant mark_cacheable! calls

Do not start a session if the current user is not logged in for public-facing pages.

Mark pages that don't care about sessions as publicly cacheable.

Keep the max age as 0 so proxies and browsers will still try to retrieve an updated version but can still fall back to the stale version if the site is down or too slow.

Fixes #9035.

* Add test that should currently fail

* Fix depth computation (will still fail if statuses have been filtered out)

* Fix handling of broken threads

* Set CSP rules in RoR's configuration

* Override CSP setting in the embed controller to allow frames

* Add animate custom emoji param to embed pages

* Rename param, use it for avatars and gifs

* Fix issues pointed by codeclimate and breaking test

* Ignore brakeman warning

* Unuse ActiveRecord::Base#cache_key

* Enable cache_versioning

* Call cache_collection

* Compensate for scrollbar disappearing when media modal visible

Make auth pages backgrounds lighter

* Fix typo

* Make embeds cacheable by reverse proxy

* Make follow button on embeds open remote follow modal

Instead of web+mastodon://, also, turn the button blue, and add a
sign up prompt to the remote follow modal

* Fix URL of "Show more" link in paginated threads (ancestors side)

Increase item limits in threads

Fix #7268

* Fix "Show more" link in paginated threads (descendants side)

Fix #7115

This also limits the statuses returned by API, but pagination is not
implemented in Web API yet. I still expect it brings user experience
better than making a user wait to fetch all ancestor statuses and flooding
the column with them.

* Allow HTTP caching of json view of public statuses

HTML views are not cached as they can contain private statuses as well

* Disable session cookies for ActivityPub json rendering of public toots

* Encode custom emojis as resolveable objects in ActivityPub

* Improve code style

- Use statuses controller for embeds instead of stream entries controller
- Prefer /@:username/:id/embed URL for embeds
- Use /@:username as author_url in OEmbed
- Add follow link to embeds which opens web intent in new window
- Use redis cache in development
- Cache entire embed

* Process Create / Announce activity in FetchRemoteStatusService

* Use activity URL in ActivityPub for reblogs

* Redirect to the original status on StatusesController#show

* Improve webfinger templates and make tests more flexible

* Clean up AS2 representation of actor

* Refactor outbox

* Create activities representation

* Add representations of followers/following collections, do not redirect /users/:username route if format is empty

* Remove unused translations

* ActivityPub endpoint for single statuses, add ActivityPub::TagManager for better
URL/URI generation

* Add ActivityPub::TagManager#to

* Represent all attachments as Document instead of Image/Video specifically
(Because for remote ones we may not know for sure)

Add mentions and hashtags representation to AP notes

* Add AP-resolvable hashtag URIs

* Use ActiveModelSerializers for ActivityPub

* Clean up unused translations

* Separate route for object and activity

* Adjust cc/to matrices

* Add to/cc to activities, ensure announce activity embeds target status and
not the wrapper status, add "id" to all collections

and atom feed generation to some extent, as well as the way mentions are
stored