WIP: Merging front and back end work streams

Tasks:

- [x] Copy over @Demwunz's stylesheets and scripts
- [x] Configure simple_form to attach expected CSS class
- [x] Fix date inputs to match GDS standard, i.e. multipart select > multipart text input
- [x] Refactor CSS so the HTML requires the least amount of additional classes etc, i.e. choose different selectors 
- [x] Radio buttons with massive hit areas
- [x] Style non simple_form elements
Changed the classes for the GDS date pattern cc @Demwunz