Set the mail username optional in setup

During the setup process email informations were asked:
  - Driver
  - Host
  - Username
  - Password

In some situations the username is not useful because the Cachet's
host may be configured to forward email to a server.

The problem is the username was required, so we had to set a username
and then update the .env file to remove it.

To fix this problem, the mail username has been set to optional in
the setup. So if someone needs a username it still can use this field,
and otherwise people can let it empty.

See: #3244

app/Http/Controllers/SetupController.php

@@ -194,7 +194,7 @@ class SetupController extends Controller
             return $input->env['mail_driver'] === 'smtp';
         });
 
-        $v->sometimes(['env.mail_address', 'env.mail_username', 'env.mail_password'], 'required', function ($input) {
+        $v->sometimes(['env.mail_address', 'env.mail_password'], 'required', function ($input) {
             return !in_array($input->env['mail_driver'], ['log', 'smtp']);
         });