Added drag-and-drop sortability option to AV arrays

Added option to create an AV array where you can drag and drop the values instead of having to use swaps.

- Requires RubaXa's [Sortable.js](https://github.com/RubaXa/Sortable)
- Works on touch devices
- Sortable arrays are similar to [this](https://trak.cs.hut.fi/sort/exercise/1)

Is it OK that this is part of JSAV, or do you think it should be an extension?