Fix unrecognised user/post mentions (#71)

* fix: Use proper mentionsUsers serializer
The only reason we haven't seen any issues with this up until now is because we have not has to include the relationship in any responses.

* fix: User/Post mentions showing as deleted after saving
The reason this happens is because the modified relationship is not refreshed, so the formatter doesn't recognize the new mentions as valid.

extensions/mentions/src/Listener/UpdateMentionsMetadataWhenVisible.php

@@ -55,6 +55,7 @@ class UpdateMentionsMetadataWhenVisible
     protected function syncUserMentions(Post $post, array $mentioned)
     {
         $post->mentionsUsers()->sync($mentioned);
+        $post->unsetRelation('mentionsUsers');
 
         $users = User::whereIn('id', $mentioned)
             ->get()
@@ -69,6 +70,7 @@ class UpdateMentionsMetadataWhenVisible
     protected function syncPostMentions(Post $reply, array $mentioned)
     {
         $reply->mentionsPosts()->sync($mentioned);
+        $reply->unsetRelation('mentionsPosts');
 
         $posts = Post::with('user')
             ->whereIn('id', $mentioned)