Allow serving static resources from cdn

Since recently we have support to specify a different url for binaries to make it possible to load them from a CDN: HSTTWO-2863

The same feature would be very nice to have for (static) resources like css and javascript that are now served directly by the container.

This could be configured by setting a property on a host (channel) and optionally via a mount or sitemap. The same configuration would be nice for the binaries which is now set through a property "binaries.prefix" in the hst properties file.

The properties could be something like:
resources.url
binaries.url

Although giving them the same name might be slightly confusing since the default for resources.url would be "/" and for binaries "/binaries"

Example configs:
resources.url = / => default
resources.url = http://cdn.com/ => use cdn url
resources.url = http://cdn.com/static/ => use cdn url with prefix '/static/' (aka http://cdn.com//static/js/jquery.js)
resources.url = https://cdn.com/ => use https
resources.url = //cdn.com/ => use client side http/https