Modal updates

## Breaking changes
Syntax for using modals has been updated. See usage example below:

```html
<a href="http://www.example.com" data-modal-href="#modal--example">Click!</a>

<div data-modal id="modal--example" role="dialog">
  <!-- content -->
</div>
```

## Changes
- Modals now keep state when they're closed.
- Fixes issue with scrolling & forms in modals on iOS devices.
- Improved performance when opening and closing modals.
- Better SEO and screen reader support.


For review: @DoSomething/front-end.