Expose the customer email in the order edit form by default

Assigning to Ryan for review :).

I believe we also need to update this for the default configuration imported on installation, otherwise it will not apply to default orders. I'd also propose shortening the help text a little:

Cannot be directly set for draft orders; updates based on checkout input or user email address.

Updated the MR via GitLab's fancypants web IDE. I'm so hip.

Merged, thanks!