Support for `/epic` quick action

Summary

In &11138 (closed), we created quick actions to achieve parity with available quick actions related to epics in the new epics work items experience. However, rather than implementing epic for use on issues, we implemented /set_parent so that it can be allowed across work item types (e.g., on tasks to set parent issue, on KR to set parent objective, on issue to set parent epic, on epic to set parent epic).

Unfortunately we did not create a migration strategy for /epic which can leave some users believing this feature doesn't exist and/or break saved templates that involve the quick action.

Proposal

In this issue, we need to add alias support for /epic quick action.

🎗️ Noting that #476602 may be an intersecting change for this work.