pluginified most of hasFave, getFaves and related calls

The code is now more event-driven when it comes to rendering notices
and their related HTML elements, since we can't have direct calls from
core to a plugin.

lib/activitymover.php has a function to move a Favorite activity which
will not happen now. The move must be pluginified and performed as an
event which plugins can catch on to.

plugins/GroupPrivateMessage/GroupPrivateMessagePlugin.php

@@ -230,7 +230,7 @@ class GroupPrivateMessagePlugin extends Plugin
      *
      * @return boolean hook value
      */
-    function onStartIntepretCommand($cmd, $arg, $user, &$result)
+    function onStartInterpretCommand($cmd, $arg, $user, &$result)
     {
         if ($cmd == 'd' || $cmd == 'dm') {