Single input submit fix

Having a single input field meant some smart answers failed to submit on pressing enter, as described here: http://colinmccann.com/code/aberrant-behavior-with-single-input-forms-in-ie7

This is to move the 'next' parameter off the submit button to ensure it is always sent through.

This issue is recorded in this ticket: https://govuk.zendesk.com/tickets/20007