Shift DirectGov to be managed by transition

This pull request shifts direct.gov.uk to be managed by transition.

The tasks that have occurred as part of this
* mappings have been established in [the transition tool](https://transition.production.alphagov.co.uk/sites/directgov) for all content assets hosted on [directgov](http://www.github.com/alphagov/assets-directgov).
* rules have been replaced with individual mappings
* no consideration has been given to page furniture or image assets hosted on directgov. [example](http://www.direct.gov.uk/govuk-logo.png)

This PR should not be merged until the challenges of [this story](https://www.pivotaltracker.com/story/show/73977438) has been considered / solved.