CANVAS-159 create high-resolution assets for the canvas theme

I don't have source files for the backgrounds, so this just covers
the logos for now. That's probably enough.

add 2x version of sfu-canvas-logo.png

add 2x version of sfu-logo.png

use 2x versions of images
Merge pull request #23 from grahamb/canvas159-retina

CANVAS-159 create high-resolution assets for the canvas theme