Fix indentation to use spaces, not tabs
Consistent use of full stop

Up to now, the organisation status description text only had one full
stop unless 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.