Select only one role between user and administrator

[maximemf]

Is your feature request related to a problem? Please describe.

In the USERS admin menu, when editing a user, you can grant it both the standard user role and the administrator role.

That doesn't make much sense, because the administrator role already includes the user role, there is no difference if a user has only the admin role or both the user and admin roles, so it can be confusing for the users.

Describe the solution you'd like

You should only be able to select one role, user or admin, but not both. So I should have a dropdown list with both roles and just select one or the other.

Other possibility: a simple admin checkbox, checked to grant admin role, unchecked to remove admin role.