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.