DHCPv6 server edit when DHCPv6 Relay enabled
DHCPv6 server edit when DHCPv6 Relay enabled

Part of Redmine #6997
This implements the similar behaviour as for the DHCPv4 server in Pull
Request https://github.com/pfsense/pfsense/pull/3271

brunostein/pfsense,dennypage/pfsense,pfsense/pfsense,dennypage/pfsense,NOYB/pfsense,ptorsten/pfsense,NewEraCracker/pfsense,brunostein/pfsense,phil-davis/pfsense,dennypage/pfsense,phil-davis/pfsense,NewEraCracker/pfsense,pfsense/pfsense,dennypage/pfsense,NewEraCracker/pfsense,ptorsten/pfsense,NewEraCracker/pfsense,jxmx/pfsense,phil-davis/pfsense,pfsense/pfsense,brunostein/pfsense,NOYB/pfsense,pfsense/pfsense,jxmx/pfsense,NOYB/pfsense,phil-davis/pfsense,jxmx/pfsense,ptorsten/pfsense