Introducing workers

This PR contains a number of pieces of work
- Adding sidekiq
- Renaming ManualServiceRegistry to OrganisationalManualServiceRegistry
- Introduce a Publishing Workers Layer to abstract workers from the controller which in turn can call the   services and perform them asynchronously.

Would it be better to have the async publish service return a manual object rather than having to call the show service here?

Note: this is a comment for discussion, rather than a reason to not merge this PR.