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
Yes, it's one of those thing you don't expect to change. A strategy would definitely be useful if churn was high. But would be happy to use a strategy later.