feat(collapse): make collapse work with bootstrap3

Closes #1240

If starting out collapsed, the expand animation would jump since the `initialAnimSkip` was still `true`.
fix(collapse): Prevent consecutive transitions & tidy up code

Previously, there would be a case where the two transitions would run
as the cancel would asynchronously invoke the reject handler of the
transition which would set the currTransition to undefined even when
the currTransition has been replaced with a new transition.
feat(accordion): convert to bootstrap3 panel styling