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/Http/SlugManager.php

@@ -0,0 +1,27 @@
+<?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\Http;
+
+use Illuminate\Support\Arr;
+
+class SlugManager
+{
+    protected $drivers = [];
+
+    public function __construct(array $drivers)
+    {
+        $this->drivers = $drivers;
+    }
+
+    public function forResource(string $resourceName): SlugDriverInterface
+    {
+        return Arr::get($this->drivers, $resourceName, null);
+    }
+}