Resolve "Merge requests and issues don't show for all subgroups"

What does this MR do?

Fix display of issues and MRs in subgroups when the current group has no issues or MRs. Also fixes the count in the sidebar.

Are there points in the code the reviewer needs to double check?

The existing behaviour was really confusing - issues used a helper, but MRs used a method defined on Groups::ApplicationController (but only used for the merge_requests action).

I just put them both in memoized helper methods instead.

Why was this MR needed?

Bug in 10.5!

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #43510 (closed).