Fixed absolute path issues to allow use of virtual host monster and proxies...

I have worked my way through the notebook templates and updated links to be mostly absolute and be virtual host monster aware so that the sage notebook can be properly proxied behind Apache2 or nginx.  It is now possible to run multiple sage server instances on one domain name.  I think I have found all the things that were broken, but this does need testing by others.  This does not change local usage although the path names are slightly different.