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