* Do not serve account actors at all in limited federation mode

When an account is fetched without a signature from an allowed instance,
return an error.

This isn't really an improvement in security, as the only information that was
previously returned was required protocol-level info, and the only personal bit
was the existence of the account. The existence of the account can still be
checked by issuing a webfinger query, as those are accepted without signatures.

However, this change makes it so that unallowed instances won't create account
records on their end when they find a reference to an unknown account.

The previous behavior of rendering a limited list of fields, instead of not
rendering the actor at all, was in order to prevent situations in which two
instances in Authorized Fetch mode or Limited Federation mode would fail to
reach each other because resolving an account would require a signed query…
from an account which can only be fetched with a signed query itself. However,
this should now be fine as fetching accounts is done by signing on behalf of
the special instance actor, which does not require any kind of valid signature
to be fetched.

* Fix tests

* 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

Fix #6856
Fix #6951

* make accounts_controller_spec DRY

* Add blocked user spec

* Show pinned statuses only in the top of the profile page

* Refactor AccountsController#show_pinned_statuses?

* Allow multiple pinned statuses to be shown and make them be ordered by pinned date

* Set timestamps NOT NULL

* Make single-line pinned_statuses

* Spec for pinned_statuses

* Remove redundant empty line

* Improve webfinger templates and make tests more flexible

* Clean up AS2 representation of actor

* Refactor outbox

* Create activities representation

* Add representations of followers/following collections, do not redirect /users/:username route if format is empty

* Remove unused translations

* ActivityPub endpoint for single statuses, add ActivityPub::TagManager for better
URL/URI generation

* Add ActivityPub::TagManager#to

* Represent all attachments as Document instead of Image/Video specifically
(Because for remote ones we may not know for sure)

Add mentions and hashtags representation to AP notes

* Add AP-resolvable hashtag URIs

* Use ActiveModelSerializers for ActivityPub

* Clean up unused translations

* Separate route for object and activity

* Adjust cc/to matrices

* Add to/cc to activities, ensure announce activity embeds target status and
not the wrapper status, add "id" to all collections

* Introduce recent scope to Status and StreamEntry

Introduce recent scope to Status and StreamEntry as Account has.

* Cover AccountsController more in AccountsController

* Add routing specs for accounts followers and following actions

* Use more restful route naming for public account follow pages

Moves two actions:
- accounts#followers to accounts/follower_accounts#index
- accounts#following to accounts/following_accounts#index

Adds routing spec to ensure prior URLs are preserved.