[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.

[ogra1]

Well, technically the user should not need to bother in the snap world ... but practically I see that it allows to set a timer for updates which is even useful in the snap world... for the general update check I'd just show a note in the UI that the snap system cares for this on its own, the timer part of it would need some patching though to go conform with the snapctl way of setting timers ...