Fix issue 1399 to update deployment settings for svn mission

The svn mission has been generating 500 errors when a user tries to submit a diff. I believe that adding the svn-mission subdomain to ALLOWED_HOSTS may solve these issues. 

Reasoning
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A subdomain, svnadmin.openhatch.org, to the deployment_settings file so that the svn
mission would continue to work after adding CloudFlare service.
https://github.com/openhatch/oh-mainline/commit/e7ac7b7aab48162686ede7ae74e4ad3ec0e5b070

The svn repo prefix would now include the subdomain svn-mission
SVN_REPO_URL_PREFIX = 'svn://svn-mission.openhatch.org/'

As part of the migration to version 1.5, ALLOWED_HOSTS was set.
https://github.com/openhatch/oh-mainline/commit/e6a78168bfa27dbca07940c9eb7fb9a792850cf5

I believe that the 500 error may be solved by adding the subdomain svn-mission to the
ALLOWED_HOSTS setting.