Admin has ordering functionality for images within a portfolio gallery

Requires #17.

- [x] JS for drag-and-drop reordering stuff on the fly
- [x] The image's positions are stored within the expanded `flickr_images_flickr_tags` (?) table instead of the `flickr_images` table
- [x] The PortfoliosController has an edit and an update functionality that allows for images to be ordered within each gallery