Set .gitignore to ignore test files

Files in /dist are used for Tenon.io. Ignore these.
Change Start now button snippet to be a link

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

This will require some additional Javascript so that links which look
like buttons can be activated using the space key, this will later be
provided by the govuk_template
Update dependencies