* Change domain blocks to automatically support subdomains

If a more authoritative domain is blocked (example.com), then the
same block will be applied to a subdomain (foo.example.com)

* Match subdomains of existing accounts when blocking/unblocking domains

* Improve code style

* Fix URI not being sufficiently validated with prefetched JSON

* Add additional id validation to OStatus documents, when possible

An explicit error allows user agents to know the error and Sidekiq to
retry.

* If an OStatus message contains nsfw hashtag, mark it as sensitive

Undo parts of #7048

* Put nsfw hashtag on OStatus messages if they have any media

* Fix code style issues

* Revert "Fixes/do not override timestamps (#7331)"

This reverts commit 581a5c9d29.

* Document Snowflake ID corner-case a bit more

Snowflake IDs are used for two purposes: making object identifiers harder to
guess and ensuring they are in chronological order. For this reason, they
are based on the `created_at` attribute of the object.

Unfortunately, inserting items with older snowflakes IDs will break the
assumption of consumers of the paging APIs that new items will always have
a greater identifier than the last seen one.

* Add `override_timestamps` virtual attribute to not correlate snowflake ID with created_at

* Do not override timestamps for incoming toots

* Remove every reference to override_timestamps

Statuses are now created with the announced publishing date
and are only pushed to timelines if that date is at most
6 hours earlier than the time at which it is processed.

Fix #6821

* fix validation error (media only status)

* Incorporating review suggestions

* Reflect similar fix to OStatus side

* Fix not to include media in transaction

* Restore the limit of the number of media

* Fix not to return nil

Time window of 6 hours

* Skip ActivityPub Announces of non-public objects

* Skip OStatus reblogs of non-public statuses

- Rename Mastodon::TimestampIds into Mastodon::Snowflake for clarity
- Skip for statuses coming from inbox, aka delivered in real-time
- Skip for statuses that claim to be from the future

Additionally, ActivityPub::FetchRemoteStatusService no longer parses
activities.
OStatus::Activity::Creation no longer delegates to ActivityPub because
the provided ActivityPub representations are not signed while OStatus
representations are.

* Avoid races in incoming OStatus toots processing

* oops

* oops again

* Custom emoji

- In OStatus: `<link rel="emoji" name="coolcat" href="http://..." />`
- In ActivityPub: `{ type: "Emoji", name: ":coolcat:", href: "http://..." }`
- In REST API: Status object includes `emojis` array (`shortcode`, `url`)
- Domain blocks with reject media stop emojis
- Emoji file up to 50KB
- Web UI handles custom emojis
- Static pages render custom emojis as `<img />` tags

Side effects:

- Undo #4500 optimization, as I needed to modify it to restore
  shortcode handling in emojify()
- Formatter#plaintext should now make sure stripped out line-breaks
  and paragraphs are replaced with newlines

* Fix emoji at the start not being converted

Currently, private / direct posts via OStatus from AP compatible instance will be dropped due to failing to fetch AP version.

So this fallbacks to OStatus handling:

* when failed to fetch ActivityPub version
* when status is neither :public nor :unlisted

* Wrap methods of ProcessFeedService::ProcessEntry in classes

This is a change same with 425acecfdb15093a265b191120fb2d4e4c4135c4, except
that it has the following changes:

* Revert irrelevant change in find_or_create_conversation
* Fix error handling for RemoteActivity

* Introduce Ostatus name space