If the id of a `.formulaequationinput input` element contains a special
character, then the selector for $preview was silently failing to match
the element, because no escaping was happening.
This fixes the issue by escaping the id before passing to the jQuery
selector function. CSS.escape is the ideal method, but this isn't
present in IE or Edge, so we use a fallback borrowed from the new
jQuery.escapeSelector method.
9c4b458d2a