Improve ActivityPub representations (#3844)

* 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

app/views/accounts/show.activitystreams2.rabl

@@ -1,9 +0,0 @@
-extends 'activitypub/types/person.activitystreams2.rabl'
-
-object @account
-
-attributes display_name: :name, username: :preferredUsername, note: :summary
-
-node(:icon)   { |account| full_asset_url(account.avatar.url(:original)) }
-node(:image)  { |account| full_asset_url(account.header.url(:original)) }
-node(:outbox) { |account| api_activitypub_outbox_url(account.id) }