[5.x] Fix read-only state of roles and groups fields

[aerni]

I noticed that the user_roles and user_groups fields in the user blueprint aren't displayed as read_only even if a user doesn't have the assign roles and assign user groups permissions. This PR fixes that. It's only a UI fix, as the UsersController already guards the fields from being updated without the appropriate permissions.

Before:

After:

I actually went a step further and decided to hide the fields instead of making them read-only. I think this is more appropriate. Users without permission to change their role and group shouldn't be able to see those fields at all. It's just confusing.