Reduce wptreport storage requirements

[jgraham]

We cache downloaded wptreport.json files for some time after initially downloading them, since in some cases we use them multiple times e.g. both for doing a metadata update and also for doing results comparions for notification pushes.

We now seem to bet 8Gb of uncompressed wptreport files from a full wpt run. That makes it easy to run out of disk space. We also don't have any insight into how useful the cache is.

We should:

• Reduce the number of pushes for which we retain logs
• Consider storing the logs in a compressed format
• Ensure that these logs are removed by a cleanup script and consider running the cleanup script automatically if we detect low disk space.