This fixes compatibility with django 1.8.
RemovedInDjango110Warning: Session verification will become mandatory in
Django 1.10. Please add
'django.contrib.auth.middleware.SessionAuthenticationMiddleware' to your
MIDDLEWARE_CLASSES setting when you are ready to opt-in after reading
the upgrade considerations in the 1.8 release notes.
If user wants to register a subdomain, he must prove he is owner of
2nd-level domain. He must create a file with random name generated by us
in webserver's document root in 2nd-level domain. Then we trying to
fetch this file. If it success, registration gets accepted.
