Add backend support for bookmarks

Bookmarks behave like favourites, except they aren't shared with other
users and do not have an associated counter.

app/serializers/rest/status_serializer.rb

@@ -8,6 +8,7 @@ class REST::StatusSerializer < ActiveModel::Serializer
   attribute :favourited, if: :current_user?
   attribute :reblogged, if: :current_user?
   attribute :muted, if: :current_user?
+  attribute :bookmarked, if: :current_user?
   attribute :pinned, if: :pinnable?
 
   belongs_to :reblog, serializer: REST::StatusSerializer
@@ -71,6 +72,14 @@ class REST::StatusSerializer < ActiveModel::Serializer
     end
   end
 
+  def bookmarked
+    if instance_options && instance_options[:bookmarks]
+      instance_options[:bookmarks].bookmarks_map[object.id] || false
+    else
+      current_user.account.bookmarked?(object)
+    end
+  end
+
   def pinned
     if instance_options && instance_options[:relationships]
       instance_options[:relationships].pins_map[object.id] || false