Deletion of a required role (Admin or Reader) on user edit page sometimes only works as expected after page reload

## Current Behavior
A user needs to have at least the Reader or Admin role. When a user has e.g. only the Reader role and an admin tires to remove this role on the user edit page, we do not allow the deletion and display a warning. This does not work as expected under the following circumstances:

User 1 = normal user with Reader role
User 2 = admin user

1. User 2 adds Admin role for User 1
2. User 2 tires to delete Reader role for User 1
3. User 1 still has the Reader role and User 2 sees warning "Roles must at least contain Admin or Reader role."

Removing the Reader role only works after a page reload.

## Expected Behavior
Step 2 should be possible and we should not display the warning.

* Graylog Version: 4.0rc1