Candidate fix for #1812

If the user-agent is iOS, then:

1. Add an extra class so that people can choose to opt in to the workaround by adding a style.
2. Adjust the heights for the modal and backdrop.
3. Scroll the window to the top. If you open a modal via a trigger at the bottom of a long page, the modal might open above the viewport, so all you see is the backdrop and no other indication a modal is open.

Style needed to complete the workaround:

```css
.modal-ios {
	position: absolute;
	overflow: visible
}
```

You could add the style inline, but probably better to just offer a hook for people to opt-in to.