Container scanning - show signature badge next to image name

For the container registry page, the signature is currently shown as a separate "image":

If this signature image exists, hide it and instead show a Signed badge next to the matching image:

There can also be multiple signatures for an image. If so, design and implement a UI to show the additional signatures (no designs are available, come up with something reasonable).

Implementation Plan

• Create a feature flag for this feature. It cannot go live until backend starts sending the signature data.
• Modify app/assets/javascripts/packages_and_registries/container_registry/explorer/pages/details.vue to hide the signature image and show the signed badge. Use the example data format from this comment.