Accessibility fixes

Accessibility fixes emerging from the high priority recommendations of the Digital Accessibility Centre report on the Stamp Duty Calculator.

Changes:

1. Heading hierarchy (h1, h2, etc) reorganised to cascade properly.
2. 'Detail' links, i.e. links that make visible hidden items, have had notices added to make clear that they do so.
3. Various adjustments to the flow of the forms to make it more logical for screen readers. These include taking the slider out of tabindex, and flipping the panels so that the calculation comes after the form in the DOM.

The dough slider presented some issues here. Would be great to see a real-world accessible dough slider somewhere if I can.