Deal with browser cache invalidation on debusine upgrades

We have observed a UI glitch after upgrade that fixes itself when forcing a reload of site assets browser-side.

This is an issue that may get worse in the future as we become more UI heavy.

The main solution that has been proposed is to look into ManifestStaticFilesStorage

A plan B from that is to add ?cachetag=$GITCOMMIT to static asset URLs

A possible crazy plan C is to add the deployed git commit to the static files path, and keep N last deployed collectstatic directories around.

Plan A looks like the standard Django way of doing what we need.