Ensures email alert is only sent once to the Email Alert API

https://www.pivotaltracker.com/story/show/79010378

In order to prevent the unlikely-but-possible case where a message is delivered multiple times
on the queue for a single change to a piece of content, we set a key in Redis with an agreed expiry time of three months. If a lock key exists for a message, the Email Alert API is not notified.

Authored by @dp6ai and @pedrosmmoreira.
(I think I recall sidekiq doing automatic retry with backoff anyway, so that should just come out automatically.)