Rewrite to improve the hints/fact sheets code.

We decided to rename STACK hints -> fact sheets, to avoid confusions
with Moodle interactive behaviour hints.

The display is now done using Moodle's print_collapsible_region
function.

The code that ensures that facts get displayed been moved out of
CAStext, and into maths display area.

The code that controls how the hints are displayed has been moved into
th renderer.