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
>> (0 / 0.0) || 0.0
=> NaN

(Or am I missing the point?)