Cookbook Notifications

:fork_and_knife: This starts to implement email notifications for cookbook followers when a new version is published. This also moves the creation of seed data for `mail_view` from the `MailPreview` class into `seeds.rb` developers will need to run `rake db:seed` in order to use `mail_view` but this should be a development prerequisite recommendation.

- [x] Implement cookbook version notification email queuing with Sidekiq.
- [x] Implement ability for users to opt out of email notifications.