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/controllers/accounts_controller.rb

@@ -16,7 +16,9 @@ class AccountsController < ApplicationController
         render xml: AtomSerializer.render(AtomSerializer.new.feed(@account, @entries.to_a))
       end
 
-      format.activitystreams2
+      format.json do
+        render json: @account, serializer: ActivityPub::ActorSerializer, adapter: ActivityPub::Adapter
+      end
     end
   end