Ability to track workflow initiators

[ri-pandey]

Currently, Bioloop does not track workflow initiators. It would be useful to do so in cases where a workflow is launched from the UI (i.e. by a user). All other workflows are assumed to be created by svc_tasks.

Also tighten the access restrictions around the Delete operation - anyone with the operator role should only be able to delete workflows that were initiated by them. admin role should be able to delete all workflows.

We have dataset_audit table where currently something like this is tracked. But only when someone launches a delete dataset workflow. This can be extended for other types of workflows.

[ri-pandey]

Closed by #266