Fix #44. Remove sudo permissions for non-admins.

I took the easy way out on this one. I just updated user.can_sudo?() to
only allow admins (is_admin? == true) to sudo, rather than allowing
period administrators to sudo into their coaches' or interns' accounts.

I don't expect that anyone will complain about this because I suspect
that none of them have been using this feature. But I am going to leave
all of the surrounding structure in place so that I can go back and add
this feature if people actually liked it.