Use GovUK Elements css and js

- Reuse css/js from govuk elements
- The css/js is included via govuk_elements_rails gem
- Override with our own css when required
- Change show/hide sections to use details/summary elements and details polyfill js
- Use selection-buttons js to highlight focused element
- Deleted obsolete css files
Prefer single-quoted strings when you don't need string interpolation or special symbols.