Associate default user address in pre-transition callback

Two advantages:

  1. Centralizes this logic, as we want this to happen whether you create an order from frontend, backend or API.
  2. Waits until the last possible moment to assign an address and will thus get the most recent default address, rather than the one at the time the cart was created.

This is as an alternative to [#4789](https://github.com/spree/spree/pull/4789) and based off the IRC discussion with @JDutil, @athal7 and @huoxito. cc/ @nhippenmeyer
The `check_box` form helper allows you to specify different checked and unchecked values; personally, I think that's more confusing and we are better off with an extra line of code.