Use DFP API to Identify Sponsored Articles

This change integrates DFP data to use for making decisions about what to show.
The admin server caches data from DFP in json in S3 every half an hour.
This data is made available to other services by a common agent that makes a scheduled read of the cache.

The DFP API client could do with some more refactoring, but will do that later.

/cc @kenlim @gklopper