Epics from all parent groups should be fetched in issue sidebar

Summary

The epic dropdown in issue page only fetches the epics belonging to the direct parent of the issue's project.

The correct behavior should be fetching all the epics from the parent groups of the project.

A few customers have reported having problems assigning epics to issues due to this bug.

Internal slack discussion for reference: https://gitlab.slack.com/archives/C4XFU81LG/p1623042808377300

current behavior	correct behavior

Steps to reproduce

1. Prepare a group (root) and a subgroup.
2. Create an epic belonging to the root group.
3. Create an issue under any project belonging to the subgroup.
4. Go to the created issue's page and check out the epic dropdown.

Example Project

https://gitlab.com/gitlab-org/plan-stage/sub-group/sub-project/-/issues/8

The epic dropdown in the issue page doesn't display any epics belonging to either plan-stage or gitlab-org parent groups.

Possible fixes

Use the root group as the fullpath when fetching epics. #332820 (comment 594093445)