New group membership confirmation in the DB

[adehnert]

At the moment, the membership confirmation page new groups use is ​https://asa.scripts.mit.edu:444/applications/startup/confirm-membership.py. This is unfortunate, since the dropdown list doesn't actually live update from the DB. We should move it into the DB, because (a) we already have semi-similar functionality in the DB, (b) it'll be relatively easy to live-update that list, and (c) it'll be a good opportunity to add exciting features like "email asa-groups when the fifth person confirms".