this adds a new page with instructions for sharing user invites
if a user clicks the link for one of their own invite codes they are
directed to a page with clear instructions for it's use.
if a user clicks another users link they are redirectec to their account
settings witha flash telling them they do not need to register.
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
Matt Baer
Rob Loranger