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
@JDutil The entire file was following the old syntax, so I changed it throughout the entire file to stay consistent.

What's best practice with this in Spree in general? Do I follow current convention within the same file, or convert everything to new syntax when I touch it?