Use drivers for display names, add display name extender (#2174)

* Deprecate GetDisplayName event

* Add interface for display name driver

* Add username driver as default

* Add code to register supported drivers / used driver as singletons

* Configured User class to use new driver-based system for display names

* Add extender for adding display name driver

* Add integration test for user display name driver

* Add frontend UI for selecting display name driver

framework/core/src/Extend/User.php

@@ -0,0 +1,38 @@
+<?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;
+
+class User implements ExtenderInterface
+{
+    private $displayNameDrivers = [];
+
+    /**
+     * Add a mail driver.
+     *
+     * @param string $identifier Identifier for display name driver. E.g. 'username' for UserNameDriver
+     * @param string $driver ::class attribute of driver class, which must implement Flarum\User\DisplayName\DriverInterface
+     */
+    public function displayNameDriver(string $identifier, $driver)
+    {
+        $this->drivers[$identifier] = $driver;
+
+        return $this;
+    }
+
+    public function extend(Container $container, Extension $extension = null)
+    {
+        $container->extend('flarum.user.display_name.supported_drivers', function ($existingDrivers) {
+            return array_merge($existingDrivers, $this->drivers);
+        });
+    }
+}