Re-add util class and mark it as deprecated

This would be the only breaking change in beta.12. Let's stick to our
backwards compatibility promise / intentions as much as possible, even
if we assume the class has not been used anywhere.

This BC layer will be removed again for beta.13.

Refs #1975.

src/Util/Str.php

@@ -0,0 +1,23 @@
+<?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\Util;
+
+use Illuminate\Support\Str as Laravel;
+
+class Str
+{
+    /**
+     * @deprecated
+     */
+    public static function slug($str)
+    {
+        return Laravel::slug($str);
+    }
+}