Add workflow to tags

https://www.pivotaltracker.com/story/show/67644106

This adds a state machine to the `Tag` model, which allows a tag to change from `draft` to `live`. This also changes the default state of a tag to 'draft', rather than 'live'. These changes support the introduction of a workflow into Panopticon for publishing tags. 

This is a breaking change, as the change in the default state for a tag will break lots of existing behaviour in tests and importers. To minimise the chance of this breaking change blocking the repo, I have the Panopticon workflow branch ready to become a pull request, and a branch on the Content API to update all the tests to explicitly use live tags.