Have an option to send email notification with real room messages

Altering the scripts/utils/send-unread-notification-email.js script to be able to take the last 3 messages from each room and send them.

Testing strategy

1. Add the Mandrill webapp prod secrets to your local webapp .env file
   • Don't forget to re-export the .env into your updated environment, https://gitlab.com/gitlab-org/gitter/webapp#configure-service-secrets
   • You can skip this step if you want to use the fake mailer
2. node ./scripts/utils/send-unread-notification-email.js --username MadLittleMods --room-uri madlittlemods/duck-wings