Expose contact connected state to REST API

See merge request !1260

Fix overflow in expiry warning code, bypass expiry code in release builds

See merge request !1259

Check server socket is bound

Closes #1744

See merge request !1258

Use colours from style guide

See merge request !1257

Enable LAN plugin by default, move default settings to constants

See merge request !1256

Merge plugin-toggles feature branch

See merge request !1255

Let's not break too many things at once.

Remote addresses are checked for acceptability before connecting.

This allows IPv6 to be used when providing an access point.

This is just a precaution - on all devices I've tested, the wifi
interface is the only one with a SLAAC address.

This ensures connections are closed when the user disables a transport.