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.

src/Discussion/Discussion.php

@@ -23,7 +23,7 @@ use Flarum\Notification\Notification;
 use Flarum\Post\MergeableInterface;
 use Flarum\Post\Post;
 use Flarum\User\User;
-use Flarum\Util\Str;
+use Illuminate\Support\Str;
 
 /**
  * @property int $id