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.
Awesome, just as a side note, this would also allow you different settings for different environments since you could change the config in `qa.rb`, `staging.rb`, `production.rb`, etc.