Refactor collapsible animation

[aatkin]

This kind of duplicates what was previously handled by the classes/ids. Would be good to refactor this into the atom completely (with CSS animation for opening/closing). Doesn't have to happen now, but we can make sure we have an issue for it.

Originally posted by @Macroz in #2951 (comment)

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Currently rems.collapsible/block is using Bootstrap collapse via Javascript API (https://getbootstrap.com/docs/4.1/components/collapse/#via-javascript).

#2951 introduces atom-based state for tracking internal show/hide state that toggles between "hideable collapsed" and "hideable expanded" content. Bootstrap uses jQuery and Javascript to handle dynamic element animation, which maybe needs to be re-created if the functionality is refactored into the atom.