Change svn mission domain name in production

We recently switched to CloudFlare accelerating port 80 and 443
on openhatch.org, which had the unfortunate results that since the
CloudFlare service does not forward the svn port to our server,
users were getting errors trying to 'svn checkout svn://openhatch.org/...'.

This patch changes the domain name we use to be svn-mission.openhatch.org.

I have created the corresponding CloudFlare DNS entry, and tested that
svn checkout svn://svn-mission.openhatch.org/paulproteus/trunk
does the right thing.