Add gzip support for heatmap downloading

[meilinger]

Looks like it'd be best to put a CloudFront distribution in front of the S3 bucket that houses heatmaps in order to add gzip since it'd be responsive to browser capabilities (vs ALWAYS sending gzipped content in the option described below).

The other option is gzipping the heatmaps before uploading to S3 and adding the Content-Encoding: gzip header (see https://medium.com/@graysonhicks/how-to-serve-gzipped-js-and-css-from-aws-s3-211b1e86d1cd ).