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

framework/core/views/frontend/content/index.blade.php

@@ -7,7 +7,7 @@
         @foreach ($apiDocument->data as $discussion)
             <li>
                 <a href="{{ $url->to('forum')->route('discussion', [
-                    'id' => $discussion->id . (trim($discussion->attributes->slug) ? '-' . $discussion->attributes->slug : '')
+                    'id' => $discussion->attributes->slug
                 ]) }}">
                     {{ $discussion->attributes->title }}
                 </a>