Captured from @Jdlrobson in https://github.com/wikimedia/pixel/issues/108

https://pixel.wmcloud.org/reports/desktop/index.html currently shows the latest master against the release branch.
It would be useful to generate and preserve reports for each release so that in the event of unbreak now/regressions we can look back at history to identify where the issue was introduced.

e.g. https://pixel.wmcloud.org/reports/desktop/1.39.0-wmf.27 would show us the difference between 1.39.0-wmf.27 and 1.39.0-wmf.26.

THis could be done with a cronjob, but ideally would happen by dynamically setting the directory of the report on each run.