Rewriting code for notice representation

Getting rid of NoticeListItemAdapter, putting more into ActivityHandlerPlugin
and relying on plugins to handle rendering code of the content. This gives us
a lot more structure and consistency in notice structure and allows activity
plugins to stop rendering certain kinds of notices more easily.

There should also be a property for an ActivityHandlerPlugin class to avoid
rendering notices in the ordinary stream, so we don't have to overload stuff.

plugins/Favorite/FavoritePlugin.php

@@ -289,6 +289,19 @@ class FavoritePlugin extends ActivityHandlerPlugin
         }
     }
 
+    public function showNoticeListItem(NoticeListItem $nli)
+    {
+        // pass
+    }
+    public function openNoticeListItemElement(NoticeListItem $nli)
+    {
+        // pass
+    }
+    public function closeNoticeListItemElement(NoticeListItem $nli)
+    {
+        // pass
+    }
+
     public function onAppendUserActivityStreamObjects(UserActivityStream $uas, array &$objs)
     {
         $faves = array();