Generate Retina versions of uploaded image thumbnails.

When fetching the thumbnail for an image, we now generate both a
standard and Retina version of the image. retina.js will display the
appropriate one when rendering. This adds a bit of nice crispness to any
thumbnails with fine detail, such as screenshots.

Tested with Firefox and Chrome with a Retina display and without.

On the Retina display, I saw a lot more detail than before.