Add a separate DRAFT_SUBMISSION event (#3075)

Fixes #3021 

NEW_SUBMISSION is getting used to notify staff members on slack and also
to notify the applicant. But for draft submission, only the applicant
should get the notification, there shouldn't be any recorder activity or
slack notification.
So, added a separate event DRAFT_SUBMISSION that only sends the
notification/email to the applicant.

Co-authored-by: Fredrik Jonsson <frjo@xdeb.org>