Add an ID attribute to each question in the markup

This does two things:
- Fixes [card #7891](https://www.agileplannerapp.com/boards/187120/cards/7891)
  (Clicking on a summary error message not taking user to question being validated)
- Allows linking the 'Answer required' message text to the questions
  to which it relates
Merge pull request #143 from alphagov/give-each-question-an-ID

Add an ID attribute to each question in the markup