This includes the appmigrations table in the schema files, and inserts
the current database version when running writefreely --init-db

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

This runs database migrations and obviates the need for manually running
changes.

Ref T509