Add state for queued publishing

Now that publishing is handled by the sidekiq queue, we should handle state of a PublishTask in order to give feedback to the user and provide a mechanism for tracking errors with queued publishes. Instead of using the PublicationLog for this we will use a stateful model 'PublishTask'. Which can be used for both error tracking purposes and editor feedback.

![Churn](http://www.quickmeme.com/img/d0/d0b970ab4510ba2e95982b6799055e7876ab9cdce48b00096450b031ce8dc529.jpg)