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
