An attempt to make the scoped smart_answers styles more explicit

Scoping means we can overide styles from static.
Making them more explicit reduces knock on effects from changes to
these classes from static.

I have added some comments to highlight some parts that I think could
do with refectoring

https://www.agileplannerapp.com/boards/105200/cards/6024
There are `h2s` for [multiple questions on a page](https://www.preview.alphagov.co.uk/employee-parental-leave/y), but they also have a [`.page-title`](https://github.com/alphagov/smart-answers/blob/make-scoped-smart-answers-styles-more-explicit/app/assets/stylesheets/smart_answers.scss#L44) class with styling, so might not be actually be used as just `h2`