MDL-77856 qtype_multianswer: Use Bootstrap Popover for subq feedback
The YUI Overlay widget encloses the subquestion feedback in a div
which causes a div element to be enclosed in the subquestion span. This
leads to an accessibility issue in terms of HTML parsing as inline
elements (span) should not contain block elements (div)
The YUI Overlay widget is also not accessible as it does not really hide
the overlay contents via aria-hidden when the overlay is not being
shown. It's better if we stop using this and use Bootstrap's
popover component which is more accessible by default.
This patch also removes module.js for the qtype_multianswer plugin as
it only contains codes related to rendering the feedback contents in the
YUI overlay widget which is no longer necessary.