Add sanity checking of JSON data

[pmac]

The JSON data should be checked before it's used. Some files have dependencies on others. For example:

• LATEST_FIREFOX_VERSION in https://product-details.mozilla.org/1.0/firefox_versions.json has to match records in https://product-details.mozilla.org/1.0/firefox_primary_builds.json
• Same for beta, aurora, ESR, etc.

We've implemented this in bedrock, but should move to this lib. It'll be a little challenging because we have to find a temporary location to which to download the files and check them before moving them into place on the FS or in the DB.