Read compressed JSON files

[gareth-j]

Compressing the exported JSON using gzip results in a ~ 75% reduction in size.

1. Do we need to compress these ourselves, or are they compressed between GitHub and the client?
2. How best to decompress, maybe using the Compression Streams API?

[gareth-j]

So looking at the amount of data transferred in the network tab of Firefox's devtools it looks like it's getting compressed automatically anyway. I'll leave this open for now to see if we can make any improvements such as setting a higher compression level than GitHub's servers.