Simplify permissions and add API to register configurable ones

Lots of thought has gone into this; it will show up later when I do the
admin permissions interface / category permissions :)

src/Support/ServiceProvider.php

@@ -5,6 +5,7 @@ use Illuminate\Contracts\Events\Dispatcher;
 use Flarum\Core\Models\Notification;
 use Flarum\Core\Models\User;
 use Flarum\Core\Models\Post;
+use Flarum\Core\Models\Permission;
 use Closure;
 
 class ServiceProvider extends IlluminateServiceProvider
@@ -90,4 +91,9 @@ class ServiceProvider extends IlluminateServiceProvider
             }
         });
     }
+
+    protected function permission($permission)
+    {
+        Permission::addPermission($permission);
+    }
 }