Fix scaling bug in scribing_canvas.js which flipped the X and Y values
Merge branch 'scribing-qns' of https://github.com/kxmbrian/coursemology.org into scribing-qns
Add canvas to individual scribing question edit page so the picture will
also load when editing an individual question.
Merge branch 'scribing-qns' of https://github.com/kxmbrian/coursemology.org into scribing-qns

Conflicts:
	app/assets/javascripts/scribing_canvas.js
Correctly displays multiple scribing questions when attempting mission

Add hidden image element to page. Edit canvas dimensions to match A4
aspect ratio.

Edit scribing_canvas.js to collect all canvas and image elements, then
create fabric.Image objects and assign them as canvas backgrounds. Image
scales are calculated before fabric.Image creation.

References Coursemology/coursemology.org#277
Merge pull request #4 from fonglh/scribing-qns

Display multiple scribing questions on mission attempt page