Added SMTP test button in the admin gui

- Added a test button for checking the e-mail settings.
- Fixed a bug with the _post JavaScript function:
  A function was overwriten with a variable and errors were not handled
correctly like a 500 for example.

src/config.rs

@@ -668,6 +668,7 @@ where
     reg!("email/verify_email", ".html");
     reg!("email/welcome", ".html");
     reg!("email/welcome_must_verify", ".html");
+    reg!("email/smtp_test", ".html");
 
     reg!("admin/base");
     reg!("admin/login");