Copy govuk_frontend_toolkit js from node module

- Update Gruntfile.js to copy files from the govuk_frontend_toolkit npm
to govuk/public/javascripts/
- Update scripts include to link to new JavaScript for radio buttons
and checkboxes
Use GOVUK.selectionButtons

- Update application js
https://github.com/alphagov/govuk_frontend_toolkit#selection-buttons
- Rename to form-chunky-labels partial to form-block-labels to follow
classnames
Add example page for radio buttons and checkboxes
Add JS to show additional hidden content

- Remove .toggle-content, replace with generic .js-hidden
- Add JavaScript to show hidden content associated with radios and
checkboxes
- Fix show/hide not working correctly for National Insurance number
example