Provide a non-deprecated way to override "style.css" using the Skin Application

Steps to reproduce:

• create a new skin page
• add a new template "style.css" using the "Add" button next to "Add a new overriding template"
• add the content from the default template on the filesystem:

  ## Get the compiled LESS code
  #set($css = $services.lesscss.compileSkinFile('style.less.vm'))
  ## Remove the useless part
  #set($index = $css.indexOf('.not-sent-to-the-browser-from-this-point'))
  #if($index >= 0)
    #set($css = $css.substring(0, $index))
  #end
  ## Remove from the CSS some rules that make IE9 crash using the WYSIWYG (XWIKI-10661)
  ## Only if the user agent contains the IE9 signature
  ## @TODO: Remove this when IE9 will no longer be supported.
  #if($request.getHeader('User-Agent').contains('MSIE 9.0'))
    ## We removes css rules like ".className:empty{...}"
    ## "(?s)" means that the expression "." matches any character, including a line terminator
    #set($css = $css.replaceAll('(?s)\.[^\{]*:empty\{[^\}]*\}', ''))
  #end
  ## Display the CSS
  $!css
  

• set the skin in administration
• open http://localhost:8080/xwiki/bin/view/Sandbox/TestPage1

Result: the page appears without a skin.