optionsToClasses: Concentrate deprecated code in a single extension
Collapsible and collapsibleset: Remove code deprecated in 1.4.0
Collapsible: Add classes option

Fixes gh-7690
Collapsibleset: Add classes option
Collapsible: Adding collapsed and heading-collapsed classes and testing
Collapsible: Begin using backcompat code
Collapsible: Update classes option when style options change