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
Add redundancy date question, update copy.
Copy and logic changes for vehicles-you-can-drive
Change option label '18 or 20' to '18 to 20'
Merge remote branch 'origin/vehicles-i-can-drive-amends'
Fix over_60 calculation
Remove 'none of these' options, adjust copy
Merge branch 'master' into energy-grants-calculator
Add label for none option
Merge remote branch 'origin/energy-grants-calculator'
Create student-finance-forms.rb
Create student-finance-forms.yml
Create student_finance_forms_test.rb
Initial logic for pip-dates.
Add validation for given postcode
Copy for pip-dates checker
Merge pull request #145 from alphagov/pip_date_checker

Pip date checker
Merge remote branch 'origin/single-input-submit-fix'
Update logic for pip-date-checker.