[Snap] Implement Update Settings

[benfrancis]

In the web interface in Settings -> Updates there is a user interface which can tell the user the current version of the gateway which is running, enable them to turn automatic updates on and off, and manually check for an update to install.

This is functional on other platforms (e.g. https://github.com/WebThingsIO/gateway/blob/master/src/platforms/linux-raspbian.ts#L771), but will need re-implementing for the snap/Ubuntu Core.

May need the snap-refresh-control interface.