SA - Rent residential property

New SA: Landlord Immigration Checker.

- Built using the new Smartdown sintax
- Uses the GOV.UK's version of MapIt for validating postcodes

https://www.agileplannerapp.com/boards/105200/cards/5222


It wasn't immediately clear that was what was going on, it looked like accidental omission. 

For better readability it might be having two lists, one with correct/modern area names (city) and a second legacy list (boroughs), then when our mapit is brought up to date we can cleanly delete the legacy list.