Photo gallery enhancements

Currently the gallery block simply loads images from a collection. There are a couple enhancements that would be useful:
* Ability to re-order images in the gallery (might require changing how it works with collections sadly)
* Left right arrows to swipe through the images. Could possibly be implemented by a carousel inside the modal.
* Reduce image load glitch (currently when you close an image and open a new image, there is a delay as the browser unloads the old image and loads the new one)