fix(frontend/UsersList): ensure newly added users don't have any roles

Previously, when a project was edited, when the last user was removed,
followed by adding a different user, the roles of the removed user
would still be checked.
Now when removing a user and adding a different user, all role checkboxes
of the new user are empty.

Fixes #458

Signed-off-by: martinfrancois <f.martin@fastmail.com>