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

### Before
![](https://s3.amazonaws.com/f.cl.ly/items/020O2e3j0H3X2M0T0A09/Screen%20Shot%202014-12-08%20at%2015.11.23.png)

### After
![](https://s3.amazonaws.com/f.cl.ly/items/3Y2e0o0L2T47041Z1P1h/Screen%20Shot%202014-12-08%20at%2015.10.42.png)
if `with_title` isn't true then we'll have an empty `tr`/`th`, which isn't ideal