Store discussion slug in database table

In preparation for #646.

migrations/2016_02_04_095452_add_slug_to_discussions.php

@@ -0,0 +1,41 @@
+<?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\Core\Migration;
+
+use Flarum\Core\Discussion;
+use Flarum\Database\AbstractMigration;
+use Flarum\Util\Str;
+use Illuminate\Database\Schema\Blueprint;
+
+class AddSlugToDiscussions extends AbstractMigration
+{
+    public function up()
+    {
+        $this->schema->table('discussions', function (Blueprint $table) {
+            $table->string('slug');
+        });
+
+        // Store slugs for existing discussions
+        Discussion::chunk(100, function ($discussions) {
+            foreach ($discussions as $discussion) {
+                $discussion->slug = Str::slug($discussion->title);
+                $discussion->save();
+            }
+        });
+    }
+
+    public function down()
+    {
+        $this->schema->table('discussions', function (Blueprint $table) {
+            $table->dropColumn('slug');
+        });
+    }
+}