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
:+1:

i'd rely on [rails `submit` form helper](http://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-submit) for the button text, and remove the difference in styling, as i don't think it gives us much value. what say?