MDL-41757 quiz statistics : limit subq and variant stat rows

and print min, median and max.
MDL-43992 Javascript: Filter M.core.confirm return key submissions

The return key should perform the default action, but only if the current
selection is not of a type which has it's own events.

We apply this blanked to all anchors, buttons, and submittable inputs
including the actual 'Yes' button as the browser's own handling of the
return on the button is sufficient.

This listener is intended to perform the default action when no submittable
element is currently selected.