Make sagenb not root in prep for virtual host monster support

Converted all paths in sagenb to begin with '/sage'. TODO convert this
to something that only needs updating one place!
absolute paths and virtual host monster

Many little tweaks to make absolute paths and virtual host monster work
so that the notebook will work properly behind a proxy.  Only for twistd
so far and many worksheet commands still broken.
propogating site_name through templates and page building code to allow proxying.

Added g.site_name to the global flask page generating and templating
code to allow proxying to a site of a specific name.  Still need to
update the notebook docs to reflect this.