24 queries of `ci_job_artifacts` when accessing merge request page

Summary

@smcgivern found

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18092 generates 24 queries like SELECT "ci_job_artifacts".* ...

https://gitlab.slack.com/archives/C4X68LQ5B/p1522774367000519

It seems we have some sort of N+1 problem somewhere related to Ci::JobArtifact.

/cc @ayufan