Store discussion slug in database table

In preparation for #646.

framework/core/src/Api/Serializer/DiscussionBasicSerializer.php

@@ -34,7 +34,8 @@ class DiscussionBasicSerializer extends AbstractSerializer
         }
 
         return [
-            'title' => $discussion->title
+            'title' => $discussion->title,
+            'slug'  => $discussion->slug,
         ];
     }
 

framework/core/src/Core/Discussion.php

@@ -22,10 +22,12 @@ use Flarum\Event\DiscussionWasRestored;
 use Flarum\Event\DiscussionWasStarted;
 use Flarum\Event\PostWasDeleted;
 use Flarum\Event\ScopePostVisibility;
+use Flarum\Util\Str;
 
 /**
  * @property int $id
  * @property string $title
+ * @property string $slug
  * @property int $comments_count
  * @property int $participants_count
  * @property int $number_index
@@ -432,4 +434,17 @@ class Discussion extends AbstractModel
     {
         static::$stateUser = $user;
     }
+
+    /**
+     * Set the discussion title.
+     *
+     * This automatically creates a matching slug for the discussion.
+     *
+     * @param string $title
+     */
+    protected function setTitleAttribute($title)
+    {
+        $this->attributes['title'] = $title;
+        $this->slug = Str::slug($title);
+    }
 }

framework/core/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 ?: '-';
+    }
+}