[feat request] reorder challenges/categories

hope we can have a page where we can reorder the challenges and categories by dragging them around
see http://sortablejs.github.io/Sortable/

my first thought was that we can add a new column "priority" to the challenges table, and dragging challenges on the frontend changes that value in the submitted form.

Review the frontend entry points for challenge and category management, the challenges table, and the submitted form handling; the issue names no files or tests. Compare the requested drag-and-drop behavior with SortableJS, then trace how ordering is persisted; done means both challenges and categories can be reordered and the order survives submission.

Written by the indexing model from the issue text.