WIP: Merging front and back end work streams

Tasks:

- [x] Copy over @Demwunz's stylesheets and scripts
- [ ] Configure simple_form to attach expected CSS class
- [ ] Fix date inputs to match GDS standard, i.e. multipart select > multipart text input
- [ ] Refactor CSS so the HTML requires the least amount of additional classes etc, i.e. choose different selectors 
- [ ] Hook up JS progressive disclosure