[Snap] Implement Local Domain Settings

[benfrancis]

In the web interface at Settings -> Domain there is a check box to toggle mDNS/DNS-SD broadcasts on and off, and a text field to set the local hostname.

On other platforms this allows users to enable and configure a .local domain.

E.g. see https://github.com/WebThingsIO/gateway/blob/master/src/platforms/linux-raspbian.ts#L492