* Add semi-support for Video/Image objects in ActivityPub

Video and Image objects will create corresponding status records
with manually crafted text contents (title + URL)

* Extract html-url-finding logic into JsonLdHelper

* Fallback to id when url missing, extract supported object types

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.

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.

* Process Create / Announce activity in FetchRemoteStatusService

* Use activity URL in ActivityPub for reblogs

* Redirect to the original status on StatusesController#show

* 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