[Admin] Allow Admins to assign manager role from UI

[mhudson]

Users with Admin role should be able to assign :manager role to arbitrary users.

If :manager has a shared email with another non-manager person, we must ensure the manager role still works when that user is selected using switch-user. That is, we should be checking for the role using selected-user, not current-user.

Use of the rollify helper is_manager (and is_admin) is desirable.