Add several unique indexes/constraints

* Convert all invoice references to be OneToOneField. This will cause
  django to create a UNIQUE constraint on the foreign key column (which
  works fine since it's NULLable in the cases needed). And there should
  never be two objects (e.g. registrations) referencing a single invoice
  without going through an intermediate object (like a bulkpay object).

* Create unique indexes on (attendee, conference) and (email, conference)
  in confreg. This has been code-level-enforced for several years since
  multiregs were created, but has not had constraints due to old data
  being incorrect. And it's happened at least once that this caused a
  race condition - so create the expected constraints. This step may
  require manual cleanup of databases if the migration fails with a
  unique violation.

* Create unique indexes on (conference, trackname) and (conference, roomname).

* Set some default ordering to be more reasonable.

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