Improve/expand Supermarket migration metrics

The previous metrics gave a helpful but simplistic overview of the status of the import by comparing raw Supermarket counts to raw Community Site counts. The new metrics show how many records of various types were imported into Supermarket and roughly how many records could potentially be imported from the Community Site.

In the case of cookbook versions and consequently supported platforms, the comparison will not be 1-1. This is because there are a dozen or so cookbooks on the community site which have duplicate cookbook versions with the same version number. These duplicate cookbook versions also have duplicate associated platform versions records. Because we don't import the duplicate records, the numbers don't match up exactly.

This is what the new output looks like:

```json
{
  "migration": {
    "platforms": {
      "community_site": 14939,
      "supermarket": 14861
    },
    "users": {
      "community_site": 48997,
      "supermarket": 48997
    },
    "followings": {
      "community_site": 7850,
      "supermarket": 7850
    },
    "deprecated_cookbooks": {
      "community_site": 17,
      "supermarket": 17
    },
    "cookbook_versions": {
      "community_site": 5387,
      "supermarket": 5374
    },
    "cookbooks": {
      "community_site": 1339,
      "supermarket": 1339
    },
    "collaborators": {
      "community_site": 446,
      "supermarket": 446
    }
  },
  "total_users": 48997,
  "total_follows": 7850,
  "total_cookbooks": 1339,
  "total_cookbook_versions": 5374,
  "total_cookbook_downloads": 610524
}
```