Add `searchBy` helper & `SidebarItemEpicsSelect` placeholder component

What does this MR do?

This MR backports frontend changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14763 to CE.

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

https://gitlab.com/gitlab-org/gitlab-ee/issues/6878