Integrates with email alert API

* Sends alert notifications to email alert API via http.
* Adds Sidekiq as queueing mechanism for sending alert notifications

Story: https://www.pivotaltracker.com/story/show/79009772

Authored by @elliotcm, @pedrosmmoreira, @dp6ai.
This looks fine, but we need to put the config in place for the preview environment before merging.