Allow artefacts to be tagged with organisations

We'd like to be able to tag artefacts with organisations related to the content. Allowing content to be tagged with organisations makes it possible to introduce scoped search functionality in the future. 

This branch bumps govuk_content_models to introduce the new tag type `organisation`, and adds a corresponding field to the "Tags" section of the artefact form. 

This also adds a class to import organisations from the Whitehall API, which can be run from a Rake task. I've included the Whenever gem so that this task can be scheduled to run daily (pending a further change to alphagov-deployment).

It's important to note that the organisation tag type is not intended to denote ownership of the content by an organisation. However, as a further piece of work we do intend to tag publishing organisations for content in Whitehall as part of the Panopticon registration process.