Adds a 'none' env option to WEBROOT_REDIRECT so that no `location /`
configuration is written to nginx.conf.
This is useful for setting up Mailu and Mailman where we override the
root to proxy to the mailing list server instead. Without this change
the nginx container will not start, or for 1.7 users can set their
WEBMAIL_PATH to / with no webmail to get the same results.
This fix means that future users don't have to choose between webmail
and a root override and makes the configuration intention clear.
e7caff9811