Adds an /xdomain_proxy.html endpoint that serves
the proxy file from the xdomain library. This
allows IE9 users to iframe in the proxy page
to simulate a cross-domain request with cookies.
When configured, set an additional cookie with the CSRF
token for use by subdomains.
The cookie can have a different name than the default
CSRF cookie, preventing conflicts between cookies
from different domains (e.g. ".edx.org", "courses.edx.org",
and "edge.edx.org").
The new cookie is included only on the enrollment API
views so that the scope of this change is limited
to the end-points that require cross-domain POST requests.
