Add empty locale extender to clear cache properly on enable/disable.

See https://github.com/flarum/core/pull/2020#issuecomment-616149583

extensions/mentions/extend.php

@@ -40,6 +40,8 @@ return [
         ->belongsToMany('mentionsPosts', Post::class, 'post_mentions_post', 'post_id', 'mentions_post_id')
         ->belongsToMany('mentionsUsers', User::class, 'post_mentions_user', 'post_id', 'mentions_user_id'),
 
+    new Extend\Locales(__DIR__.'/locale'),
+
     function (Dispatcher $events, Factory $views) {
         $events->listen(WillSerializeData::class, Listener\FilterVisiblePosts::class);
         $events->subscribe(Listener\AddPostMentionedByRelationship::class);