Update client version.
Remove unused stuff.
Make Invite ancestor groups by inviter.

GAE transactions only support queries within the same ancestor
group.  This will make sure we notify everyone who was invited
before the EC2 instance for this inviter came up, avoiding race
conditions.

Giving the same guarantees for old invites would be more
involved and pointless anyway, since people invited through
old lantern-controllers have already received their installation
e-mail.  We still try and e-mail them so they get a chance to
upgrade, but we'll notify everyone with old clients through
other means eventually.