Invite friends by email

A page where a user can invite a new friend to join the local service. The user adds their friend's email(s), then an optional brief personal message.

The personal message is added to a full instructions email (including registration link) which is sent to the user's friend. When the friend registers, the user is automatically subscribed to that friend's notices (...and vice versa?).

We need a place in the DB to store invitations. user ID, email address. We just check that table when a new registration comes in, and add a subscription automatically.

We also need a way to say that a user already exists in the system, and do an automatic subscription if that's the case.