Use a single combined Rummager request for search.

There are a couple of outstanding problems here:

  * We are taking the "departments-policy" results key and translating
    it to "government" to avoid breaking styling, JavaScript and tests
  * The search tests need factoring out, at least into functional and
    integration tests.
Don't pass empty strings to search.

If `organisation_slug` or `sort` are passed as empty strings to
Rummager, it'll reject them as invalid values.
Change old "government" tab name.

This makes the markup more self-consistent and lets us remove some
special cases from the controller.
Document why we can't use Rummager to look up tabs.

I found this out the hard way.
Merge pull request #518 from alphagov/combined-search

Combined search