GitLab

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)
![Screen_Shot_2016-06-21_at_13.16.44](/uploads/db0d3ed8c8b90fafc8dbf1644c2354b0/Screen_Shot_2016-06-21_at_13.16.44.png)

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style...