Move styles to smart answers and scope them within smart_answers class

• added smart_answers.scss for smart answers scoped styles
• move styles from application to smart_answers.scss
• add colours and shims to application from FE toolkit

https://www.agileplannerapp.com/boards/105200/cards/6024
Merge pull request #1216 from alphagov/scope-styles-to-smartanswers

Move styles to smart answers and scope them within smart_answers class