Create skin file for turtle

Ideally we shouldnt have any checks in turtle.js for if (skin.id == 'elsa' or 'anna'). This gets us a step towards that by having a skin.js file for turtle. In addition, I moved a couple things that were in the base skin but specific to anna/elsa into the turtle skin file. This file behaves similarly to maze's skin.js file.

I also moved a couple anna/elsa images from common_images into the skin directory.
Haha, I like it :)

Fine the way it is, though a simple alternative would also be nesting the `CONFIGS` object within the `load` function and call `assetUrl()` on the values directly.