print all questions at once

fixes #87

elements with `visible: false` are now given a class `invisible` instead of having `display: none` applied. This class shunts them off the screen instead of actually hiding them, so MathJax can do typesetting on them.
That lets us show all questions, instead of just the current one, in the print stylesheet.

Since all maths is typeset at the start, it can slow things down a lot, so it might be worth making the "Loading..." thing wait until MathJax is finished.
Merge branch 'master' into print-all-questions
make MCQ checkboxes in revealed answer work

fixes numbas/editor#196