Allow caching of generated js/css files

Allow client caching of dynamically generated css and js files.
Default headers disable explicitly caching of application pages, but
these generated files don't change usually between page loads.
By disabling default headers, the cacheability is defined by the server
configuration.

The included resources are referenced by a URL query built with a
parameter that differentiates content that may have changed based on
current user, project, language, etc. This parameter would force a
reload in case the content may be different from the cached version.

Fixes: 0023324