Update staging and production email delivery

* Use the Mail gem's interceptor feature.
* Separate SMTP from intercepting responsibilities.
* Set up SMTP for production and staging.

http://robots.thoughtbot.com/post/40822987615/delivering-all-email-from-staging-to-a-group-email
Use recipient_interceptor gem

As suggested during code review by Joe:
https://github.com/thoughtbot/suspenders/pull/145/files#r2765329