Epic dropdown in issues filter bar is very slow for groups with many epics

Go to https://gitlab.com/gitlab-org/gitlab/-/issues and select the Epic filter.

This is very slow because we load all epics in the parent group.

Our filter bar supports DropdownAjaxFilter which passes a search param so that we can limit the results we return. This is being used for the users dropdown. So this shouldn't be too hard to implement.