Use Laravel's slugger for basic transliteration

This is better than the current system, as it adds transliteration rules
for special characters, rather than just throwing all of them away.

For languages that cannot be transliterated to ASCII in a reasonable
manner, more possible improvements are outlined in #194.

migrations/2016_02_04_095452_add_slug_to_discussions.php

@@ -7,9 +7,9 @@
  * LICENSE file that was distributed with this source code.
  */
 
-use Flarum\Util\Str;
 use Illuminate\Database\Schema\Blueprint;
 use Illuminate\Database\Schema\Builder;
+use Illuminate\Support\Str;
 
 return [
     'up' => function (Builder $schema) {