User mapping - Epic and EpicIssue work item

!148671 (merged) modified the EpicsPipeline and IssuesPipeline to be compatible with WorkItems. However, the epic and work item's authors are not being mapped to the original author.

I'm not sure if this is a bug or if it's something intentional.

If it's a bug, we should map them to placeholder users and allow them to be reassigned.