disabled interactive widgets should indicate disabled state with cursor change

[Jeff Witt]

Feedback summary

Disabled interactive things in Paragon generally have a lower-contrast appearance and don't take focus, which are OK signifiers but not super obvious in some scenarios (or with some moderate visual impairment, or low-quality LCD displays).

Improvements

There is a standard mouse pointer type
cursor: not-allowed;
that we're currently not using. I suggest we add this cursor type to any control in disabled state.
https://developer.mozilla.org/en-US/docs/Web/CSS/cursor