[Content]: `orderCompletedEmail` added to `Order` element

[nfourtythree]

Product

Commerce

Description

In the upcoming release of Commerce the Order element is getting a new property of orderCompletedEmail this property is populated when an order completes. It is a snapshot of the customer's email at this moment in time. This property is essentially "information-only" in the fact that it is not used for any of the email sending functions or anything.

As part of this change email address on the order is now kept in sync with when a user edits their email address. Previously there was some "lag" with this which was causing redundant inactive users to be created and causing potential issues with email addresses becoming "reserved".

Related PR craftcms/commerce#3180