Some cleanup work in the e-mail settings page resulted in the wrong

keyword argument being used when sending a test e-mail. We were passing

email_from and not from_email. We didn't have any unit tests

covering this, and it wasn't caught in development.
This change fixes up the keyword argument and adds a test suite. It also

improves upon the e-mail's subject, body, and error handling. The new

content more explicitly lists the product in the summary, adds a

confirmation that things look good in the body, includes recipient

information in the logs, and an error message in the message shown in

the UI.

Unit tests pass.