Prevent turning off filter in Web AppBuilder

[RandalGreene]

In Web AppBuilder for ArcGIS Online provide the ability to prevent turning off the filter widget. This would support workflows where the underlying dataset is too large to display all features.

It would require two changes:

- An option to disable the on/off toggle at the top of the filter widget.

- An option to remove the Empty/Null option from a value picklist for a filter criterion.

Thanks!