Add API endpoint for a group issues

What does this MR do?

Adds a API call for issues by group.

What are the relevant issue numbers?

Closes #17818 (closed).

• CHANGELOG entry added
• Documentation created/updated in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/api/issues.md
  • Be sure to add a note that says when was introduced this new endpoint (most probably 8.10.0)
  • Be sure to document that the labels filtering is different than the other endpoints for now
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together