Allow use of personalized from address for mailer

Admins can set `CRONUT_MAILER_EMAIL` to override the from address with one that
matches their organization rather than use the default `cronut@harrys.com`.

Sets a single, canonical location for the mailer from address, making it easier to change it application-wide in the future.
Yeah, exactly my thought. I'd much prefer to have to copy environment variables around than to have to deploy separate branches for staging vs. production environments. Thanks for the feedback.