Message queue implementation

Put a message on the queue when content is created or updated.
We (myself, Rich Bolton, and @danielroseman) were just talking about this.  What we were proposing is that we define the publishing-api to be idempotent.  This means that the transactional nature doesn't matter so much.  If an update is written to mongo, but then fails to get sent to rabbitmq, we can return a 500 error, and it's up to the publishing app to resubmit the message.

In order to achieve this we're also going to define that all consumers of the queue should process things in an idempotent manner.