Local Jami user discovery

It would be really great to be able to discover other Jami users on the local network, for example through zeroconf (https://en.wikipedia.org/wiki/Zero-configuration_networking) and / or multicastDNS (https://en.wikipedia.org/wiki/Multicast_DNS).

This would be very useful for local mesh networks such as Freifunk (https://freifunk.net), because Ring would not depend on an internet connection any more.

There was already such an issue, but it was flagged as "zombie" and closed: https://git.ring.cx/savoirfairelinux/ring-project/issues/289.