Include epics from subgroups

What does this MR do?

When listing epics in a group, include also epics from all group's subgroups. Also by_iids filter is removed from EpicsFinder because filtering by iid (which is scoped to single group) doesn't make sense since subgroups are included too.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #4301 (closed)