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
- Change .panel-indent border colour to use $border-colour
- Create new details test page at /examples/details-summary
- Change elements example to match IER’s “Help with nationality”
Can we split out work on indented panels into a separate commit? It will make it easier to see these changes as separate from the details/summary work.