Postcode value object.

It's a thin wrapper around UKPostcode that uses the formatting behaviour
we want: always normalise valid postcodes but leave invalid ones
untouched.

It also adds a `#==` method to make testing a little easier.

We didn't want to monkeypatch UKPostcode to add this behaviour.
Add ability to update a loan's postcode via a DataCorrection.
CfeUsers can make loan data corrections.
Let me use PresenterFormatterConcern for this.