Fix for OVPMS-1500 Reversal of supplier invoice produces incorrect balance

git-svn-id: svn://svn.openvpms.org/openvpms/vpms/branches/1.8-dev@6278 9e4fa352-aafb-0310-9071-df624684024b
Fix for OVPMS-1462 Check for unique user name before saving users

git-svn-id: svn://svn.openvpms.org/openvpms/vpms/branches/1.8-dev@6279 9e4fa352-aafb-0310-9071-df624684024b