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 commit 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 commit does *not* attempt to add in any additional ARIA signalling, etc
Tests no longer look for '.link-right.restart'

- Previously our tests were looking for a `<th>` with `'.link-right.restart'`. These classes have been removed and so now we can just inspect the done questions section of the markup for the presence of the link
- Fixes test errors
Modify the layout of the 'Previous Answers' section

- Addresses the previous concerns around `with_title` logic. Now if there's no title, there's no heading
- Moves the title of the section into an `<h3>`, and restyles the 'start again' link, keeping it closer to the original design, but meaning that there's no semantic issues with the table headings, as was previously signalled.
Merge pull request #1342 from alphagov/refactor/previous-answers-heading

Change table heading to span all columns