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/models/concerns/account_interactions.rb

@@ -164,6 +164,10 @@ module AccountInteractions
     status.proper.favourites.where(account: self).exists?
   end
 
+  def bookmarked?(status)
+    status.proper.bookmarks.where(account: self).exists?
+  end
+
   def reblogged?(status)
     status.proper.reblogs.where(account: self).exists?
   end