This adds any OAuth login buttons to the invite signup page, stores the invite code for the flow duration, and associates the new user with it once successfully registered.
It enables invite-only instances with OAuth-based registration.
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
