Use regex for username validation

Laravel's alpha_dash rule allows unicode letters including those with inflections, leading to issues like #832. As per discussion in #557, we are sticking with ASCII-only usernames for now.

framework/core/src/Core/Validator/UserValidator.php

@@ -45,7 +45,7 @@ class UserValidator extends AbstractValidator
         return [
             'username' => [
                 'required',
-                'alpha_dash',
+                'regex:/^[a-z0-9_-]+$/i',
                 'unique:users,username'.$idSuffix,
                 'min:3',
                 'max:30'