Improve styling of multiple question smart answers

* Style the 'question page' title, ensure it's indented
* Make previous answers heading separate from 'done-questions' block,
  which is inherited from static, avoid messing with it.
* Add some margin between questions to make grouping clearer
* Ensure theres margin between questions and the previous questions block,
  regardless of its position (top or bottom)
Merge pull request #1126 from alphagov/multiple-question-layout

Tidy multiple question layout in smart answers