MDL-44940 quiz editing page problems from MDL-43749.

* If the quiz starts with an empty page, you could not move the first
question up onto it.
* Adding page breaks in the middle sometimes added them in the wrong
place.
* Adding a new page at the end is problematic, and here we fix it as
much as possible, by letting you move the last question down to create a
new page. This needs MDL-40987 to be fixed properly.