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.

removing addition of EM advocates group through auth migrations, adding new auto-generate auth migration

updating query and forms to remove module-level database queries; some resulting changes to view logic for handling labels were also required

Django 1.11.5 upgrade

See merge request !5