Add a workflow to publish tags

https://www.pivotaltracker.com/story/show/67644106

This changes Panopticon to add a workflow to publish tags. 

The content models bump (based on changes in alphagov/govuk_content_models#198) makes all tags draft by default, and adds a state machine allowing the tag to be published. Draft tags are now marked in the list with a pink 'draft' label, and then can be published from their edit pages. 

The state of an artefact for a tag (eg. a specialist sector) is kept in sync with the tag, so we won't ever have a 'live' artefact associated with a 'draft' tag. Additionally, the artefact edit form has been updated to support tagging draft tags to artefacts. The behaviour to prevent draft tags appearing in api responses has already been added to the Content API in alphagov/govuk_content_api#194.

The test suite has been updated to use the new `live_tag` factory where `tag` was previously used. Finally, the `OrganisationImporter` has also been updated to ensure all the organisation tags that it creates are in the `live` state.
One of the nice effects of using BasicObject is that the entire class behaves like an Artefact, even down to the methods to return the class name.

SimpleDelegator could mean that we remove a few methods, but I'm not sure that the implementation using BasicObject is particularly onerous. This was based on @evilstreak's work on `SpecialistSectorTagForm` and it would be good to keep a broadly-consistent implementation across both the Panopticon form objects.