Use Smartdown answer #humanize methods for previous answer display

Related Agile planner task: https://www.agileplannerapp.com/boards/105200/cards/6341

:exclamation: Build will not pass until the smartdown gem has been updated, new version has been published and I can bump the version in this patch. :exclamation: 
:heart: Please review https://github.com/alphagov/smartdown/pull/73 first!! :heart: