Allows orgs to be closed

This allows organisations to be marked as `closed`. That state is a sibling to `transitioning`, `exempt` etc.

If an organisation is closed an optional date can be entered to say when it was closed. Closed orgs show their closed date on their GOV.UK page, and related documents. They're excluded from the organisations homepage.

Note: it's possible this pull request may be updated soon to match some design changes.

https://www.pivotaltracker.com/story/show/52534831