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/Favorite/actions/apifavoritecreate.php

@@ -118,7 +118,7 @@ class ApiFavoriteCreateAction extends ApiAuthAction
 
         // Note: Twitter lets you fave things repeatedly via API.
 
-        if ($this->user->hasFave($this->notice)) {
+        if (Fave::existsForProfile($this->notice, $this->scoped)) {
             $this->clientError(
                 // TRANS: Client error displayed when trying to mark a notice favourite that already is a favourite.
                 _('This status is already a favorite.'),