Fix N+1 queries in group-level issue list

When loading the group-level issue list (https://gitlab.com/groups/gitlab-org/-/issues), we can see several N+1 queries. This is more apparent when you set page size to 100. The getIssuesEE GraphQL query executes 664 queries.

These are the major ones I saw:

Querying the issue's epic

Issue#to_reference