Deny Duplicate Delete Draft Creation

[CarsonDavis]

Overview

The check_prior_unpublished_update_exists function ensures that only one Update is able to exist at a time. However, this should also be implemented for Delete objects. This might be best done by creating a second function or by generalizing this function to check for delete an updates.

Acceptance Criteria

• when an unpublished delete draft exists for an object, and a user attempts to create a second delete draft, draft creation should be denied with an informative message

[praveenphatate]

Tested, and duplicate delete draft creation is allowed. The test is written in this bug-duplicate_delete