Add deprecated "user" filter for posts

In the filterer refactor for ListPostsController, the filter key was changed to `author` for consistency with the AuthorFilterGambit used in discussions. This commit adds a deprecated `user` filter back in for a release to allow for a graceful transition

src/Post/Filter/UserFilter.php

@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Post\Filter;
+
+/**
+ * @deprecated beta 16, remove beta 17. Use AuthorFilter instead.
+ */
+class UserFilter extends AuthorFilter
+{
+    public function getFilterKey(): string
+    {
+        return 'user';
+    }
+}