Support user invites
This includes:
- A new user_invites config value that determines who can generate invite links
- A new page for generating invite links, with new user navigation link
- A new /invite/ path that allows anyone to sign up via unique invite link, even if registrations are closed
- Tracking who (of registered users) has been invited by whom
It requires an updated database with writefreely --migrate in order to
work.
This closes T556