* Fix anonymous access to outbox not being cached by the reverse proxy

Up until now, anonymous access to outbox was marked as public, but with a
0 duration for caching, which means remote proxies would only serve from cache
when the server was completely overwhelmed.

Changed that cache duration to one minute, so that repeated anonymous access
to one account's outbox can be appropriately cached.

Also added `Signature` to the `Vary` header in case a page is requested, so
that authenticated fetches are never served from cache (which only contains
public toots).

* Remove Vary: Accept header from webfinger controller

Indeed, we have stopped returning xrd, and only ever return jrd, so the
Accept request header does not matter anymore.

* Cache negative webfinger hits for 3 minutes

* Fix misuse of foreign_type

* Fix use of removed "add_template_helper"

* Use response.media_type instead of response.content_type in tests

* Fix CSV export controller test on Rails 6

Rails 6 sets a "filename*" field in the Content-Disposition header to
explicitly encode the filename as UTF-8.

This changes checks the first part of the Content-Disposition header so
it matches in both Rails 5 and Rails 6.

* Fix emoji formatting with Rails 6

* Make emoji output more idiomatic and robust

* Switch from redis-rails gem to built-in Rails redis cache storage

* Add tests for some cachable responses

This only covers responses that we should have managed to make cachable
so far. It's not the case of all responses that should be cachable in
the end.

* Fix RSS feeds not being cachable

* 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