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/following_accounts_controller.rb

@@ -5,5 +5,25 @@ class FollowingAccountsController < ApplicationController
 
   def index
     @follows = Follow.where(account: @account).recent.page(params[:page]).per(FOLLOW_PER_PAGE).preload(:target_account)
+
+    respond_to do |format|
+      format.html
+
+      format.json do
+        render json: collection_presenter, serializer: ActivityPub::CollectionSerializer, adapter: ActivityPub::Adapter
+      end
+    end
+  end
+
+  private
+
+  def collection_presenter
+    ActivityPub::CollectionPresenter.new(
+      id: account_following_index_url(@account),
+      type: :ordered,
+      current: account_following_index_url(@account),
+      size: @account.following_count,
+      items: @follows.map { |f| ActivityPub::TagManager.instance.uri_for(f.target_account) }
+    )
   end
 end