Add method for admins to sign up users

[vwbusguy]

Add method for admins and possibly event leaders to sign up users.
Users should get notified about this change.

Should be able to hook into https://github.com/vwbusguy/band-schedule/blob/master/services/resEvent.php, but should do user level validation if reserving on behalf of another user.

[vwbusguy]

I want to revisit this after #12 is in place, but the ajax service already exists for it, it just needs the UI built.