Return Epic Work Items as part of milestone actions when FF is enabled

What does this MR do and why?

Now that Epic work items can have Milestones attached, this MR returns those Epics as part of the response when the user loads the Group Milestone report page if the work_items_alpha FF is enabled.

Achieves this by overriding sorted_issue in the EE namespace and using WorkItemsFinder in place of IssuesFinder. Also some upates to the relevant HAML to get the page to load the items without breaking.

Note that there are still changes to the FE to be made for this page to be complete in #519589 (closed)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Locally, add a group milestone to some group, and ensure that at least 1 issue, MR, and Epic have that new group milestone attached.
2. Enable work_items_alpha feature flag
3. Load the group milestone page and verify that all the work items are shown, and that when you click the Epic it links to the epic page properly
4. Disable work_items_alpha, and reload the page, the epic(s) should no longer show up.

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Related to #522012 (closed)