Using _: property names is discouraged, as in the future,
canonicalization may throw an error when encountering that instead
of discarding it silently like it does now.

We are defining some ActivityStreams properties which we expect
to land in ActivityStreams eventually, to ensure that future versions
of Mastodon will remain compatible with this even once that happens.
Those would be `locked`, `sensitive` and `Hashtag`

We are defining a custom context inline for some properties which we
do not expect to land in any other context. `atomUri`, `inReplyToAtomUri`
and `conversation` are part of the custom defined OStatus context.

* 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

- Follow, undo follow
- Accept follow, reject follow
- Like, undo like
- Block, undo block
- Delete (note)
- Update (actor)