Stats frontend: better caching

[rossjrw]

After #81, the stats frontend now uses a lot of data while checking for new updates - if the stats are about 500 kB, then it can use up to 7.5 MB as it redownloads the file each minute.

Instead, better caching should be leveraged.

There must be some sort of way to get fetch() and CloudFront to play nicely with each other, letting the browser know whether the content has actually changed so that only a minimal request needs to be made.