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

tests/integration/extenders/UserTest.php

@@ -0,0 +1,69 @@
+<?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\Tests\integration\extenders;
+
+use Flarum\Extend;
+use Flarum\Tests\integration\RetrievesAuthorizedUsers;
+use Flarum\Tests\integration\TestCase;
+use Flarum\User\DisplayName\DriverInterface;
+use Flarum\User\User;
+
+class UserTest extends TestCase
+{
+    use RetrievesAuthorizedUsers;
+
+    protected function prepDb()
+    {
+        $this->prepareDatabase([
+            'users' => [
+                $this->adminUser(),
+            ], 'settings' => [
+                ['key' => 'display_name_driver', 'value' => 'custom'],
+            ],
+        ]);
+    }
+
+    /**
+     * @test
+     */
+    public function username_display_name_driver_used_by_default()
+    {
+        $this->prepDb();
+
+        $user = User::find(1);
+
+        $this->assertEquals('admin', $user->displayName);
+    }
+
+    /**
+     * @test
+     */
+    public function can_use_custom_display_name_driver()
+    {
+        $this->extend(
+            (new Extend\User)
+                ->displayNameDriver('custom', CustomDisplayNameDriver::class)
+        );
+
+        $this->prepDb();
+
+        $user = User::find(1);
+
+        $this->assertEquals('admin@machine.local$$$suffix', $user->displayName);
+    }
+}
+
+class CustomDisplayNameDriver implements DriverInterface
+{
+    public function displayName(User $user): string
+    {
+        return $user->email.'$$$suffix';
+    }
+}