[meta] Track public/private file issues

I just want to keep tract of potential changes to the public and private file system. This is important because for a long time users have encountered problems with theme generated files, perhaps because the public files dir was not existing or has the wrong permissions etc. In ATD8 I have strongly considered doing checks and warning users of these issues but this is a broader issue for Drupal core:

#1106160: Color module uses the public file system but doesn't check if it is set