This adds a new config AllowedNetworks per device, which when set should
contain a list of network prefixes (192.168.0.0/126 etc) that are
allowed for the given device. The connection service will not attempt
connections to addresses outside of the given networks and incoming
connections will be rejected as well.
I've added the config to the normal device editor and shown it (when
set) in the device summary on the main screen.
There's a unit test for the IsAllowedNetwork method, I've done some
manual sanity testing on top of that.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4073
