Only update location when postcode changes

This is a simple fix for the [“can’t edit Newton Abbott Magistrates’ Court” bug](https://www.pivotaltracker.com/story/show/79360856): now we only ask MapIt for a location when the visiting address’s postcode has changed.

There aren’t any acceptance tests around the existing functionality, so I added a couple of very broad tests to check that things (roughly) still work.

This replaces #216, which caused many failures because of tests relying on existing (broken) behaviour.