Moved models to folder, renamed managers to tools

Tools seems to fit better since the classes were a bit of a mixed bunch
and did not always manage.
Also simplified the structure of the SlugGenerator class.
Also focused EntityContext on shelves and simplified to use session
helper.

app/Api/ApiToken.php

@@ -14,7 +14,6 @@ use Illuminate\Support\Carbon;
  * @property string $name
  * @property Carbon $expires_at
  * @property User $user
- * @package BookStack\Api
  */
 class ApiToken extends Model implements Loggable
 {