Slug Driver Support (#2456)

- Support slug drivers for core's sluggable models, easily extends to other models
- Add automated testing for affected single-model API routes
- Fix nickname selection UI
- Serialize slugs as `slug` attribute
- Make min search length a constant

src/Admin/Content/AdminPayload.php

@@ -75,6 +75,9 @@ class AdminPayload
         $document->payload['extensions'] = $this->extensions->getExtensions()->toArray();
 
         $document->payload['displayNameDrivers'] = array_keys($this->container->make('flarum.user.display_name.supported_drivers'));
+        $document->payload['slugDrivers'] = array_map(function ($resourceDrivers) {
+            return array_keys($resourceDrivers);
+        }, $this->container->make('flarum.http.slugDrivers'));
 
         $document->payload['phpVersion'] = PHP_VERSION;
         $document->payload['mysqlVersion'] = $this->db->selectOne('select version() as version')->version;