Support gzip content encoding #730

Please consider adding a header Accept-Encoding: gzip for GET requests. This will cause the server to return gzipped JSON responses, for content > 1000 bytes currently. The requests module should automatically deal with uncompressing it, so adding the header is literally all that is required.

The server supports this for all endpoints, but of course will be most beneficial for responses containing lots of data, e.g. large numbers of runs, large numbers of metrics.

(Not yet deployed on nightly, I'll update this issue once this is done).