Refactor global searches to split out group searches

Per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1578 , having the group and global search mixed up together makes the code hard to understand.

That's an EE MR, but the code will need cleaning up in CE first, then the elasticsearch path adding in EE.

It doesn't look too bad in CE: https://gitlab.com/gitlab-org/gitlab-ce/blob/v9.0.0/app/services/search/global_service.rb But in EE: https://gitlab.com/gitlab-org/gitlab-ee/blob/a49c38c65133626f7ffbe20060ebbd9f0ec2d016/app/services/search/global_service.rb

mentioned in merge request !10687 (merged)

closed via merge request !10687 (merged)

mentioned in commit nick.thomas/gitlab-ee@3fa4d8ab

mentioned in issue #31319 (moved)

mentioned in merge request !14785