Revert "credential: stop using deprecated property for email"

This reverts commit 149fdd1d.

This did not support the case of updating old emails for which
url_string was set, it was setting coordinate_text and causing
emails to have two properties ( url_string to the old value
and coordinate_text to the new value ).

See discussion on nexedi/erp5@149fdd1d (comment 180744)