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

tests/integration/api/users/CreateTest.php

@@ -25,9 +25,10 @@ class CreateTest extends TestCase
         $this->prepareDatabase([
             'users' => [
                 $this->adminUser(),
+                $this->normalUser(),
             ],
             'groups' => [
-                $this->adminGroup(),
+                $this->adminGroup()
             ],
             'group_user' => [
                 ['user_id' => 1, 'group_id' => 1],