Allow customizing legal address in email footer

Default to Automattic's address, but allow customers to modify the address to their own:


We're getting feedback from customers that emails "don't feel like theirs" as we push our brands quite visibly here.

This change goes together [with updating the footer design](https://github.com/Automattic/wp-calypso/issues/92681), and in future also allowing full control over the template via template editor.

The issue names no files, tests, or entry points. Start by reviewing the related footer-design issue, #92681, and locating the email footer implementation and its existing customer settings. Done means the footer defaults to Automattic's address while allowing customers to use their own address, consistently with the updated design.

Written by the indexing model from the issue text.