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
Bump Smartdown to v0.5.0 to get Snippets
Add snippets to simple Animal example flow
Import SPL snippets
Disable interpolation/calulations in SPL

Replace %{}, which does interpolation, with !{}, so they're not run, but
we can still regex them out easily.

- None of the calculations are implemented yet
- Some of the snippets reference answers they don't _seem_ to have been
  given while getting to that outcome
- Add an example outcome using snippets

These aren't implemented yet