Banner for closed organisations

Pivotal Tracker story: 66185062
We used to be able to mark an organisation as closed, and specify any superseding organisation.
We now can now specify the reason for closure of an organisation.
The green 'closed' banner now has more meaningful content about the closure of an organisation.
Fix indentation to use spaces, not tabs
Consistent use of full stop

Up to now, the organisation status description text only had a full
stop if it contained more than one sentence.
Make helper responsible for returning safe string
Fix schema.rb
Combine closed and no_longer_exists traits

It doesn't make sense to have one without the other.
Ensure all closed orgs have a closed status

Otherwise those organisations will fail validation and also the
organisation_closed_govuk_status_description helper returns a blank
string for closed orgs that don't have a closed status.
Merge pull request #1450 from alphagov/banner-closed-organisations

Banner for closed organisations