Send Test Mail Feature (#2023)

- Add UI, backend for sending test emails
- Change mail settings endpoint to /api/mail/settings

src/Api/routes.php

@@ -309,8 +309,15 @@ return function (RouteCollection $map, RouteHandlerFactory $route) {
 
     // List available mail drivers, available fields and validation status
     $map->get(
-        '/mail-settings',
+        '/mail/settings',
         'mailSettings.index',
         $route->toController(Controller\ShowMailSettingsController::class)
     );
+
+    // Send test mail post
+    $map->post(
+        '/mail/test',
+        'mailTest',
+        $route->toController(Controller\SendTestMailController::class)
+    );
 };