Integrates with email alert API

* Sends alert notifications to email alert API via http.
* Adds Sidekiq as queueing mechanism for sending alert notifications
Addresses PR comments

* Moves loading of redis YML config to EmailAlertService::Config
* Corrects style guide violations
* Properly assigns variables to `:let` in `major_changes_spec`
* Moves `EmailAlertWorker.drain` to after block in `major_changes_spec`
Merge pull request #13 from alphagov/integrate-with-email-alert-api

Integrates with email alert API

https://www.pivotaltracker.com/story/show/79009772