Protect requests to /data-sets with a secret token

Backdrop calls the /data-sets endpoint to query metadata about data
sets. This includes the secret bearer tokens used to write to those data
sets.

We now protect this endpoint with a secret token which is shared between
Stagecraft and Backdrop (STAGECRAFT_QUERY_DATA_SETS_TOKEN)

See https://www.pivotaltracker.com/story/show/67103208

[Delivers #67103208]
Merge pull request #75 from alphagov/secure-config-info

Protect requests to /data-sets with a secret token