Update database seeds with the dummyapiuser for development

The gds-sso gem mock MockBearerToken strategy if it cannot find a user
will call dummy_api_user.save! Sequel defaults to raising exceptions
when save fails and doesn't have a save! method, we could alias it
but adding the dummy user resolves the issue.

https://github.com/alphagov/gds-sso/blob/master/lib/gds-sso/bearer_token.rb#L56