t/devices.html: add "none" to function options when adding sensors

[Trek Hopton]

Sometimes we don’t want to have a function that modified the VidGrind sensors. Currently this is not possible via the UI on the devices page. We need to add a menu item for “none” in the sensor functions.

Currently if nothing is selected in the function dropdown, the error is shown: “sensor func missing“.

[Alan Noble]

This must be a porting problem, as the none function is supported by NetReceiver.

[Alan Noble]

Implemented in https://bitbucket.org/ausocean/iotsvc/commits/548893e8a509e5889c8c2701889e503bc14fda3d