Invitation codes should expire or be changeable

[holmesworcester]

Right now an invitation code lasts forever. Invitation codes should expire after a time set by the user. The default should be 2 weeks.

The expired onion address should be removed completely so that there is no way it could (via some bug, e.g.) point to something or reveal something.

Choices: 1 day, 3 days, 1 week, 2 weeks, 1 month, 3 months, never

A user should also be able to reset the invitation code, by creating a new onion address and forgetting the old one. This is much simpler than timed expiration so we should do it first.