In progress - sortable

I've implemented sortable [#372] (no functionality yet, just for UI testing). Although it is working as "expected" there are some issues with the interface here IMO:

* draggable nature of elements interferes with scrolling down the page
* if an element is bigger than the viewport, or you want to drag something past the end of the page, this is not possible
* dragging large elements feels confusing in general

I think we need to work the experience here and/or consider other options for ordering blocks