Implement "renamed" posts

Record when the discussion was renamed, from what, and by whom.
Information is stored in the `content` field as a serialised JSON
object because proper polymorphism will be too difficult with Ember
Data and especially when extensions try to add new post types.

ember/app/models/discussion.js

@@ -36,6 +36,7 @@ export default DS.Model.extend({
   }),
   loadedPosts: DS.hasMany('post'),
   relevantPosts: DS.hasMany('post'),
+  addedPosts: DS.hasMany('post'),
 
   readTime: DS.attr('date'),
   readNumber: DS.attr('number'),