Add `role="button"` to transaction start links

![](https://s3.amazonaws.com/f.cl.ly/items/1g3M2X3K1F1D3N1c2J1B/Screen%20Shot%202014-12-08%20at%2011.38.44.png)

The 'Start now' buttons on all start pages are often actually links styled to look like buttons. This means that to Assistive Technology (AT), which reads the document based on its HTML rather than its visuals, they are declared as something different to their appearance.

Léonie Watson suggested adding the 'role' attribute, set to 'button', which should give a better experience to users of AT

- Appropriately signals a button for users of Assitive Technology
- Adds it in as the last attribute, not sure if we care about ordering
Merge pull request #698 from alphagov/feature/add-aria-role-to-start-links

Add `role="button"` to all elements that act like a button