Role Manager / New module: Editable roles manager

[JoryHogeveen]

See filter editable_roles

It might be nice to have a simple interface where you can set the editable roles per role, only if this role is even allowed to edit users at all.

It will be important that we check all roles to make sure all editable roles are fetched.
For example.
Admin >> Can edit all
Editor >> Can edit author?
If the user has both the admin and editor roles it should still be able to edit all roles.