updating flash messaging for contact/notification creation to use django message middleware rather than session

adding optional override to Event delete() method which removes data directory and all associated GroupObjectPermission objects

Repository reorganization and port to gunicorn

See merge request lscsoft/gracedb!7

Rework migrations

See merge request lscsoft/gracedb!6

Adding temporary 'hack' for handling labels when creating an event for better backwards compatibility with older versions of gracedb-client. This should be removed in the near-ish future.