Issue #13 - Allow dragging widgets to reorder

[mblaul]

Adding drag and drop to the widget cards for reordering
Adding react-sortable-hoc for drag-n-drop features
Putting the sorting logic and components in SortableWidgetContainer
Turning the widget titles into drag handles for the cards

Sorry this took a while @emibcn, thanks again for checking in periodically. Pull this down and try it out. Let me know what you think!

Notes

• The card title is the draggable handle.
• There is no resizing in this PR unfortunately, the dnd seemed like enough for me. 🤣
• localStorage and the URL looked good to me. After moving cards around I could turn the app off and back on and their positions persisted.
• I added arrayMove with react-sortable-hoc because that's what they use in their docs. We could probably roll our own but I felt this was the simplest thing to do.

[emibcn]

Great!! Love your code: very clean, split logic, well indented and strategically commented. If I could, I'd give you a 10!

[emibcn]

Thanked again!!