Use Slug Driver system properly

[askvortsov1]

The Resource classes for user and discussion don't currently use the slug driver system properly:

• https://github.com/FriendsOfFlarum/sitemap/blob/master/src/Resources/Discussion.php#L31
• https://github.com/FriendsOfFlarum/sitemap/blob/master/src/Resources/User.php#L30

We should probably resolve the slug driver manager, and use that to fetch the proper slug driver