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/Discussion/IdWithTransliteratedSlugDriver.php

@@ -0,0 +1,42 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Discussion;
+
+use Flarum\Database\AbstractModel;
+use Flarum\Http\SlugDriverInterface;
+use Flarum\User\User;
+
+class IdWithTransliteratedSlugDriver implements SlugDriverInterface
+{
+    /**
+     * @var DiscussionRepository
+     */
+    protected $discussions;
+
+    public function __construct(DiscussionRepository $discussions)
+    {
+        $this->discussions = $discussions;
+    }
+
+    public function toSlug(AbstractModel $instance): string
+    {
+        return $instance->id.(trim($instance->slug) ? '-'.$instance->slug : '');
+    }
+
+    public function fromSlug(string $slug, User $actor): AbstractModel
+    {
+        if (strpos($slug, '-')) {
+            $slug_array = explode('-', $slug);
+            $slug = $slug_array[0];
+        }
+
+        return $this->discussions->findOrFail($slug, $actor);
+    }
+}