Goal: 

• To support our UIs here: https://www.sketch.com/s/b55e75a8-9a53-4eb0-989e-aad208521aba we need to
  • Include the last snapshot for each repo in the repository list/show apis
  • Include the added/removed counts for each snapshot in the snapshots api and the above addition to teh repository list/show apis

Acceptance Criteria:

• Get  /repositories/UUID/snapshots should include a structure like this:

• {
    uuid:  abcd-efgh,
    repository_path: /path/to/my/repo,
    content_counts: { rpm.package: 5 }
    added_counts:  { rpm.package: 4 }
    removed_counts: { rpm.package 3 }
  }

  Note that we already store content_counts, and added/removed comes from pulp very similarly, so it should be easy to add.

• get /repositories/  & get /repositories/UUID  should also include the latest snapshot for each repo:

• {
    uuid: abcd-efgh
    name: "My repo"
    url: "http://example.com/repo/
    last_snapshot: {
       uuid: abcd-efgh,  
       repository_path: /path/to/my/repo,
       content_counts: { rpm.package: 5 }
       added_counts: { rpm.package: 4 }
       removed_counts: { rpm.package 3 }
     }
  }

  We probably want to add 'latest_snapshot_uuid' to the repository_configs table to make this faster.  When create a snapshot (in dao/snapshots.go Create(), we could just update the affected repository_config to point to the new snapshot.)