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

* Forward ActivityPub creates that reply to local statuses

* Fix test

* Fix wrong signers

- 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

Requires moving Atom rendering from DistributionWorker (where
`stream_entry.status` is already nil) to inline (where
`stream_entry.status.destroyed?` is true) and distributing that.

Unfortunately, such XML renderings can no longer be easily chained
together into one payload of n items.

SerializarbleResource#as_json serializes to Symbol keyed Hash, but current
implementation of LinkedDataSignature expects String keyed Hash.

So it generates broken payload.

* Add handling of Linked Data Signatures in payloads

* Add a way to sign JSON, fix canonicalization of signature options

* Fix signatureValue encoding, send out signed JSON when distributing

* Add missing security context

* Process Create / Announce activity in FetchRemoteStatusService

* Use activity URL in ActivityPub for reblogs

* Redirect to the original status on StatusesController#show

* Fallback to OStatus in FetchAtomService

* Skip activity+json link if that activity is Person without inbox
* If unsupported activity was detected and all other URLs failed, retry with ActivityPub-less Accept header

* Allow mention to OStatus account in ActivityPub

* Don't update profile with inbox-less Person object

- Tries to avoid performing HTTP request if the keyId is an actor URI
- Likewise if the URI is a fragment URI on top of actor URI
- Resolves public key, returns owner if the owner links back to the key

* Handle duplicate ActivityPub activities

Only perform side-effects when record processed for the first time

* Fast-forward repeat follow requests

*Note: OStatus URIs are invalid for ActivityPub. But we have them for
as long as we want to keep old OStatus-sourced content and as long as
we remain OStatus-compatible.*

- In Announce handling, if object URI is not a URL, fallback to object URL
- Do not use specialized ThreadResolveWorker, rely on generalized handling
- When serializing notes, if parent's URI is not a URL, use parent's URL

* Deliver ActivityPub Like

* Deliver ActivityPub Undo-Like

* Deliver ActivityPub Create/Announce activities

* Deliver ActivityPub creates from mentions

* Deliver ActivityPub Block/Undo-Block

* Deliver ActivityPub Accept/Reject-Follow

* Deliver ActivityPub Undo-Follow

* Deliver ActivityPub Follow

* Deliver ActivityPub Delete activities

Incidentally fix #889

* Adjust BatchedRemoveStatusService for ActivityPub

* Add tests for ActivityPub workers

* Add tests for FollowService

* Add tests for FavouriteService, UnfollowService and PostStatusService

* Add tests for ReblogService, BlockService, UnblockService, ProcessMentionsService

* Add tests for AuthorizeFollowService, RejectFollowService, RemoveStatusService

* Add tests for BatchedRemoveStatusService

* Deliver updates to a local account to ActivityPub followers

* Minor adjustments

* Handle ActivityPub follows correctly

ActivityPub follows are follow-requests. Always require an Accept.
If account is not locked, auto-accept.

* Handle ActivityPub Accept/Reject-Follow

* Fix wrong method

* Fix wrong class

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests

Fixes #4496

URI.extract is too strong, not limited to URLs, matched real text.
Same issue was present in language detector.

* Fix multipoint shortcode bug

* Added testcase for composite emoji shortcode

* Add destination informations to exception on SubscribeWorker and DeliveryWorker.

* Simplify delivery error message.

* Prevent changing Exception type...

* fix typo.

* Use the same emoji data on the frontend and backend

* Move emoji.json to repository, add tests

This way you don't need to install node dependencies if you only
want to run Ruby code

* Do not raise unretryable exceptions in ResolveRemoteAccountService

* Removed fatal exceptions from ResolveRemoteAccountService

Exceptions that cannot be retried should not be raised. New exception
class for those that can be retried (Mastodon::UnexpectedResponseError)

* Add unfollow modal

* unfollowing someone

* remove unnecessary prop