webpack: fix CSS/JS files being cached forever in Safari.

Fix issue where CSS and Javascript files were permanently cached by Safari in development mode.
Safari seems to cache CSS and JS files forever, no matter what the Cache-Control header says. The
workaround is to use `Vary: *`.

config/webpacker.yml

@@ -82,6 +82,8 @@ development:
     pretty: false
     headers:
       'Access-Control-Allow-Origin': '*'
+      'Cache-Control': 'no-store, must-revalidate, private, max-age=0'
+      'Vary': '*'
     watch_options:
       ignored: '**/node_modules/**'