Rework the Previewer Cache headers

Currently the previewer sets these headers on static resources:

• on image files:

  Cache-Control: public, max-age=2592000
  

• on CSS files:

  Cache-Control: public, max-age=2592000
  

• on JS files:

  Cache-Control: private, max-age=86400
  

  or

  Cache-Control: private, max-age=2592000
  

The private attribute is problematic for proxy and the caching of the static resources.

The max-age attribute is problematic when we upgrade version.

One solution could be to use the ETag header and leverage it for the cache control.