DNS: ability to add a custom forwarder
There was a request that users should be able to add a custom forwarder. The original forwarders (google/cloudflare/cznic) should not be editable.
Api should be specified here: https://gitlab.labs.nic.cz/turris/foris-controller/blob/master/foris_controller_modules/dns/schema/dns.json#L280
Edited by Štěpán Henek