Overridden LESS templates cannot be served by the Skin Action

Steps to reproduce:

1. Do to XWiki.DefaultSkin (edit)
2. Add a new overriding template called "less/custom.less"
3. Add some content in it
4. Open the URL http://localhost:8080/xwiki/bin/skin/skins/flamingo/less/style.less.vm?colorTheme=no
   1. You get the content of style.less.vm
5. Open the URL http://localhost:8080/xwiki/bin/skin/skins/flamingo/less/custom.less -> 404

The Skin Action only supports the old Skin mechanism (where you had to modify the XWiki.XWikiSkins class) or the file system resources, but not the Overridded templates.

Because of that, if you add a LESS resource in the skin and you try to import it via the @import command on the @lessCode field of a Flamingo Theme, the live preview of that theme is broken, because it needs to access the resource via the Skin Action (because the LESS code is compiled on the browser).