Minor JS cart fixes

For reasons I can't put a finger on, Chrome (36) doesn't let images define their own width in hidden elements. A repaint fixes the layout issue, so clicking _Check Out_ broke the layout as the images became taller suddenly.

Line 594 of `ajaxify.js` forces the repaint just before we show the cart, and fixes the image layout issue. It's hacky at best.

- Also fixed jQuery selectors that were invalid in Firefox
- Removed max-height definition on all images in the grid

Fixes #167