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/Extend/ModelUrl.php

@@ -0,0 +1,54 @@
+<?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\Extend;
+
+use Flarum\Extension\Extension;
+use Illuminate\Contracts\Container\Container;
+use Illuminate\Support\Arr;
+
+class ModelUrl implements ExtenderInterface
+{
+    private $modelClass;
+    private $slugDrivers = [];
+
+    /**
+     * @param string $modelClass The ::class attribute of the model you are modifying.
+     *                           This model should extend from \Flarum\Database\AbstractModel.
+     */
+    public function __construct(string $modelClass)
+    {
+        $this->modelClass = $modelClass;
+    }
+
+    /**
+     * Add a slug driver.
+     *
+     * @param string $identifier Identifier for slug driver.
+     * @param string $driver ::class attribute of driver class, which must implement Flarum\Http\SlugDriverInterface
+     * @return self
+     */
+    public function addSlugDriver(string $identifier, string $driver)
+    {
+        $this->slugDrivers[$identifier] = $driver;
+
+        return $this;
+    }
+
+    public function extend(Container $container, Extension $extension = null)
+    {
+        if ($this->slugDrivers) {
+            $container->extend('flarum.http.slugDrivers', function ($existingDrivers) {
+                $existingDrivers[$this->modelClass] = array_merge(Arr::get($existingDrivers, $this->modelClass, []), $this->slugDrivers);
+
+                return $existingDrivers;
+            });
+        }
+    }
+}