Load json data from CVR zip file

[nealmcb]

It is more efficient and easier for the user to provide the Dominion CVR zip file (which is how we've gotten all CVRs to date) rather than individually uploading the TestBallotManifest.json (is that the CvrExport.json?), ContestManifest.json and CandidateManifest.json.
Sometimes the zips come with multiple *CvrExport*.json files since they're so big, and my audit_cvrs code just reads them all in.