Scribing questions

This feature allows teachers to have scribing questions as part of a mission, where students and teachers can draw on a canvas and submit that as an answer. Coursemology/coursemology.org#277

- Teachers may upload PDFs or images as canvas background images. Multipage PDFs are allowed and one question will be produced per page. PDFs are converted and stored as PNG.
- Background images are optional.
- Scribbles by each user may be hidden/show.

Screenshots to come!
Trailing whitespace.