Surface deduplicated size of individual image repositories in the UI

Context

We are rolling out a new version of the Container Registry in GitLab.com that includes a metadata database (&5523 (closed)). Among others, this database will allow for efficient storage usage calculations.

This issue is part of a work plan to expose the deduplicated size of individual image repositories in the GitLab UI and API (&7575 (closed)).

The Container Registry has a new API under the /gitlab/v1/ prefix, documented here. This API includes a Get repository details operation, which allows retrieving details about image repositories, including their deduplicated size.

Proposal

Leveraging on #347349 (closed), surface the size of individual image repositories in the GitLab UI.

For now, and as described in #347349 (closed), obtaining the size of image repositories requires an HTTP request against the Container Registry API. Therefore, let's start small and only display the size information when seeing the details of a single image repository (not on the repository list view).