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
This logic could definitely be clarified. Perhaps by extracting it into some form of strategy. But maybe that's some technical debt that could be logged for this project given the (perhaps) looming importance of this release.