Message queue implementation

Put a message on the queue when content is created or updated.
i'd suggest we store `queue_publisher` in the rails `config` object, so that we can make good use of environment specific configuration files to override this config: `development.rb`, `production.rb`. then we won't require an initialiser to do the same.

i've done something similar [here](https://github.com/alphagov/content-store/pull/29/files).