Show proper image ID on registry page

What does this MR do?

Display the container registry image ID (from the config blob) instead of the first image layer ID

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

Why was this MR needed?

To show proper image ID on container_registry page. This only supports manifest V2, since the manifest V1 doesn't expose information about Image ID.

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18159.

Screenshots (if relevant)

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together