Draggable helper z-index stacking context

When dragging an item between connected fsortables the dragging helper will be underneath the items of the destination fsortable.

## Need
```gherkin
As a potential user
When I check out the demo
I want to be impressed.
```

## Deliverables
```gherkin
As a user
Given that I am on the demo page
When I drag an item between connected fsortables
Then the draggable helper will be on top.
```

## Solution
Move the helper DOM element to the end of the `body` to resolve z-index stacking issues.

#### Prerequisites
- http://api.jqueryui.com/draggable/#option-helper
- http://api.jqueryui.com/draggable/#option-appendTo
- [The stacking context](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context)

#### TODO
- [x] change the `appendTo` option.