Government organisation search filtering

Add organisation filtering to the government search result tab.

In order for the UX to make sense when there are no government results, I had to force all tabs to be displayed all of the time; otherwise you could end up in a dead-end (as @markhurrell and I decided).

The list of organisations is pulled from the search index (Rummager).

~~This is deliberately turned on in production.~~
This feature is hidden behind a feature flag ``organisation-filter``. It should be turned on automatically in preview.