Ignore organisation filtering for top results.
Avoid duplicated top results in filtered searches.

Because we were using a separate, unfiltered list to extract top results from
filtered searches, the code to remove the duplicates wasn't working because it
compared results by identity. Determining result equality by whether they have
the same link solves this.
Merge pull request #356 from alphagov/unfiltered-top-results

Unfiltered top results