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.
I would love to set this as an app config rather than a general global so you could do: `Rails.config.notification_from_email`