Correct department counts on how-government-works page

Before this, all departments were being included in the counts, including
closed ones.

The organisation index is supposed to be the official list of government
departments and that uses the Organisation.listable scope, we should
use that scope for the count.
Merge pull request #1554 from alphagov/fix-dept-counts-on-how-gov-works-page

Correct department counts on how-government-works page