Epic Tree initial backend support

What does this MR do?

Add some initial backend support for new epic tree feature. Exposes a has_issues? value on an epic, as well as allows epics to be found by a specific iid

Extracted from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Closes #10795 (closed)