Account expiration does not delete contacts

[anzhieta]

Currently, contacts are stored separately from the rest of the user data and are not removed if a new account is deleted due to lack of account confirmation within 24 hours of registration.
Contact deletion was not implemented in #85 because it's trickier to implement than removal of the main account - instead of just setting an expire on the locationShare's hash key, we need to revert the account into a stub (that is, keep userId and ICanSee, throw away the rest).