Add `closed_at` datetime to organisations

This is to track when the organisation was closed. The existence of data
in this field can be used to infer whether the org is closed or not.
Add 'closed' status to `govuk_status` attribute
Allow empty alternative formats for closed orgs
Add closed_at date field to Organisation#edit
WIP for JS
Make closed_at optional

Some organisations, especially migrated ones, won't immediately have a
specific closed_at date. We should handle that.
Show closed state on Organisation#show

Renamed the external view as part of this, because its name wasn't
really relevant any more.
Exclude closed orgs from org homepage
Fix JS
Allow closed_on to be blank; default to blank
Show organisation closed at date on admin show
Merge pull request #714 from alphagov/allows-orgs-to-be-closed

Allows orgs to be closed

https://www.pivotaltracker.com/story/show/52534831