Change table heading to span all columns

As mentioned by @tombye in [our card](https://www.agileplannerapp.com/boards/105200/cards/8701) on smart answer headings, the previous table headings didn't relate to the contents of the table.

This PR fixes that by:

- Changing the table heading to span the entire row, as with https://www.gov.uk/bank-holidays
- The 'start again' link is now below the previous answers table
- This PR does *not* attempt to add in any additional ARIA signalling, etc