Add support for authority-bounded place lookups

This adds the ability to specify that a service should return places in the same local authority as the given postcode, as opposed to the geographically nearest places.  This is required for some services where you have to use the place corresponding to your council, even if it's further away (eg register offices).

https://www.agileplannerapp.com/boards/173808/cards/6033
I considered that, but there are differences between them made me stop at the time - there were enough things in flux as it was.  I'll revisit to see if it can be done cleanly.