Use ImageMagick by default, if available

Hm. So would this require including the 'imagemagick' module in Panopoly? Or is it possible to change the toolkit to Image Magick without including that module?

I'd really prefer not to add any new modules to Panopoly 1.x per #2530998: [meta] Get Panopoly 1.x into maintenance mode (so we can focus on Drupal 8). And adding a new module to your child distribution should be super easy. So, I'm marking as "won't fix", but feel free to re-open if there's a way to do this without adding a new module.

However, this is something that's definitely open for discussion with regard to Panopoly 2.x! There's a -dev version of the imagemagick module for Drupal 8, but I don't know anything about how complete the port is.