Move 'next' param to hidden field

Submit button does not pass parameter for some browsers when form is
submitted through clicking enter when in an input field

As described here:

http://stackoverflow.com/questions/8462204/why-are-forms-not-submitted-if-there-is-more-than-one-unnamed-input
Merge branch 'master' of github.com:alphagov/smart-answers into single-input-submit-fix
Merge remote branch 'origin/single-input-submit-fix'