Store discussion slug in database table

In preparation for #646.

src/Util/Str.php

@@ -0,0 +1,32 @@
+<?php
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Util;
+
+class Str
+{
+    /**
+     * Create a slug out of the given string.
+     *
+     * Non-alphanumeric characters are converted to hyphens.
+     *
+     * @param string $str
+     * @return string
+     */
+    public static function slug($str)
+    {
+        $str = strtolower($str);
+        $str = preg_replace('/[^a-z0-9]/i', '-', $str);
+        $str = preg_replace('/-+/', '-', $str);
+        $str = preg_replace('/-$|^-/', '', $str);
+
+        return $str ?: '-';
+    }
+}