Separate assets URL.

Allows serving assets on production through COOKIE-
less subdomain, which lets us to cache files on the
client's end without any issues or having to
send extra data back and forth.

Note: Selectivizr doesn't work cross-domain. Selectivizr
requires that the site and assets are hosted under
the same domain. Using a sub-domain will require
that we ditch it, or use different assets URL on IE<9.