Events with stopped workflows sometimes cannot be deleted

Steps to reproduce:
1. Ingest a mediapackage (that hasn't previously been scheduled)
2. Stop the workflow using the REST workflow endpoint
3. Attempt to delete the event

In some cases, the event cannot be deleted. Workflow details can also not be seen in the Admin UI because the workflow job is marked deleted.

Fixed by marking the job for a stopped workflow as CANCELED rather than DELETED, and other minor changes.