Allow for class customization of quizzes

[Felienne]

As originally discussed here: #1328

This idea has two parts to make quizzes and Parson's puzzles more usable and customizable for teachers:

Make quiz questions and Parsons
Firstly, we would like teachers to be able to create questions and puzzles themselves to use in their class (and optionally, donate to us)

Select quiz questions and Parsons
It would also be nice if we could allow teachers to select their own questions/puzzles from a bigger set. This does not necessarily make sense now but if we have more questions per level then that does make sense. If we have, say 50 questions in a level, a teacher might say: I want a quiz of 10 random questions shown, or I want to hand select 10 questions myself.