Legislation change

- This updates stamp duty calculator with new stamp duty bands and calculation (which now works more like income tax)
- Percentage shown to user is now a percentage of tax over property price
- Changes made to both JS and non-JS versions
- Updates relevant copy (with Welsh)
- Better cucumbers to cover JS and non-JS versions
`git show 569d331c2d06a8287c65580fa236d9bb1dd90413 -- spec/models/stamp_duty_spec.rb` although I will try again with float and see if that works