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'd suggest it's the right time to extract out a `_form.html.erb`, which saves us duplicating the fields across templates.