Api for fetching scans.

[Pivotal 67879240](https://www.pivotaltracker.com/story/show/67879240)

This adds an api with a single /latest-scan endpoint for fetching the latest scan given enough information to identify a mailing. (and a ping endpoint)

We decided, for now, to run this in the same docker container as the scan importer but as a separate process. That change is also included.