Allow admins to customize the notification email

it's weird that both the confirmation emails (to the person who signed up) and the notification emails (to the designated signup coordinator) have idential subject lines. gets confusing in your inbox.

2 approaches:

1) easy hack: just change the hard-coded subject for notifications from: "Signup confirmation for event: %title" to "Signup notification for event: %title".

2) better yet: make the subjects of these emails also configurable through the UI, with real tokens (and a better sane default).