Introduce user & global archiving controls; add unregister function

- WARNING to operators: You MUST run fixup_2.sql in order to use this and newer
  versions of the bridge.
- Archiving and full chat history fetches are now a configuration setting (and
  default to off).
- Users now have to enable archiving manually by talking to the admin user, and
  are warned about the potential privacy implications.
- Users can now completely remove themselves from the bridge, deleting all data.
- Bridge administrators can now be specified by adding an entry to the
  'administrators' SQL table with your JID. These can force-unregister specific
  users.