MDL-43768 Edit quiz: page should not reload so often

The approach is that we have a new JavaScript function
M.mod_quiz.resource_toolbox.reorganise_edit_page which, after a ajax
action, fixes up everything like page breaks, page and question numbers,
that might now be wrong.

We call that function instead of reloading the page.

Also, there are a lot more Behat tests to verify this works correctly.
MDL-47368 core JS: dragdrop click move fills the dom with divs

Every time you click the move icon, it builds a new dialogue with a list
of drop targets, but it never discards the old ones. Not only does this
fill the DOM with junk, but it also causes Behat tests to fail, since
the old dialogues come first, and are hidden, but contain links with the
same text that you are trying to click.