Support for HTML5's <details> and <summary>

- Add details polyfill to provide a fallback for support of details and
summary
- Remove old details polyfill
- Move details scss into a new file
- Create new details test page at /examples/details-summary
- Change elements example text to match IER’s “Help with nationality”
New panel styles

- Decrease border width
- Use border-colour, not panel-colour
- Remove margins on the last paragraph in a panel
Merge pull request #18 from alphagov/improve-details-summary