Restore stricter email validation

In v5.8, Laravel expanded email validation logic to closer match the RFC. This, however, allows emails that aren't conventional (for example, emails lacking a TLD). This commit changes Flarum's UserValidator to use the `email:filter` validator, which uses PHP's filter_var, and is the pre-5.8 behavior.

See https://laravel.com/docs/5.8/validation#rule-email

src/User/UserValidator.php

@@ -51,7 +51,7 @@ class UserValidator extends AbstractValidator
             ],
             'email' => [
                 'required',
-                'email',
+                'email:filter',
                 'unique:users,email'.$idSuffix
             ],
             'password' => [