Fix pixel dimensions on factlist

[caseymm]

The images are saving at low resolutions on factlist right now and need to be scaled up to match the correct dimensions for the social images (e.g. 1200x630 for facebook, 1024x512 for twitter).

[caseymm]

I pulled the latest beta version of html2canvas, which includes the ability to scale up the image. I currently have it scaled to 2, which should be retina, but still doesn't look quite right to me. It's a lot better than it was, but would like a second set of eyes on it at some point. Work is in the fb-ratio branch. Also applies to #6.