Adding full Article support

This creates a new column in the `statuses` table which keeps track of
activity_pub_type, so in the case of a Note it will be blank (the
default) and it will be a string "Article" if the received remote object
is an AP Article. There is now a bunch of special case code in the
formatters and sanitizers to handle Articles differently, as well as on
the clientside.

app/serializers/rest/status_serializer.rb

@@ -4,7 +4,7 @@ class REST::StatusSerializer < ActiveModel::Serializer
   attributes :id, :created_at, :in_reply_to_id, :in_reply_to_account_id,
              :sensitive, :spoiler_text, :visibility, :language,
              :uri, :url, :replies_count, :reblogs_count,
-             :favourites_count, :local_only
+             :favourites_count, :local_only, :activity_pub_type
 
   attribute :favourited, if: :current_user?
   attribute :reblogged, if: :current_user?
@@ -61,6 +61,10 @@ class REST::StatusSerializer < ActiveModel::Serializer
     OStatus::TagManager.instance.uri_for(object)
   end
 
+  def activity_pub_type
+    object.activity_pub_type.to_s
+  end
+
   def content
     Formatter.instance.format(object)
   end