Increase Cache-control max-age value for static files

Currently the max-age for static files is 12h which we can safely increase to days or even months as our js and css includes the hash in the filename - index.html has to be loaded every time as its name doesn't change. Changing this is a simple configuration option for flask (SEND_FILE_MAX_AGE_DEFAULT) and shouldn't be too hard in Django (for the new backend). This could also provide an additional benefit if we updated cra as new versions include vendor splitting.