WIP: Allow object store for Job Artifacts

Downside of this implementation, this would require an extra key in the gitlab.yml. However, this seems to be the best solution.

• Changelog entry added
• Documentation created/updated
• Omnibus changes (Setting of object_store key in gitlab.yml)
• Test with S3, depends on gitlab-com/infrastructure#1316
• Test with GitLab Pages

Fixes: gitlab-org/gitlab-ce#29203