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)

Start by locating the gallery block, its collection handling, and the image modal implementation in the Django project. Review how images are ordered, navigated, closed, and loaded; done means supporting reordering, left/right or swipe navigation, and a reduced delay when changing images.

Written by the indexing model from the issue text.