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/serializers/activitypub/collection_serializer.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class ActivityPub::CollectionSerializer < ActiveModel::Serializer
+  def self.serializer_for(model, options)
+    return ActivityPub::ActivitySerializer if model.class.name == 'Status'
+    super
+  end
+
+  attributes :id, :type, :total_items,
+             :current
+
+  has_many :items, key: :ordered_items
+
+  def type
+    case object.type
+    when :ordered
+      'OrderedCollection'
+    else
+      'Collection'
+    end
+  end
+
+  def total_items
+    object.size
+  end
+end