Move show file code owners when viewing file to CE

Support for code owners was added by https://gitlab.com/gitlab-org/gitlab-ee/issues/6961 to gitlab-ce~2933322 in preparation for suggesting merge request approvers based on code owners https://gitlab.com/gitlab-org/gitlab-ee/issues/5382. Although merge request approvals are not available in GitLab CE, it is still useful to see this information when viewing files and we should move this feature to GitLab CE.

We should move the parts of Code Owners support that are not Approvals related in to gitlab-ce

Proposal

Move https://gitlab.com/gitlab-org/gitlab-ee/issues/6961 to GitLab CE

Links / references