WIP: Include container repositories in storage statistics

What does this MR do?

Adds missing storage statistics for container images, as a follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754

Since the metrics need to be fetched via the container registry API, this is implemented as a cronjob which runs daily.

This development is sponsored by @siemens (/cc @bufferoverflow)

Are there points in the code the reviewer needs to double check?

I'm not sure how to handle the API versioning (doc/development/api_styleguide.md doesn't say anything about it), do I need to update the code in lib/api/v3 as well?

Why was this MR needed?

Administrators/group owners should have an overview about storage used for container images.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #27929 (moved)