Downweight gov results to improve top 3 results

The top 3 results are picked by combining all results and picking the three top
scorers. The problem with this is that the scores aren't really comparable. For
many searches, such as "tax disc", the top results are all publications, even
though the most relevant and useful result is from mainstream: "Car tax: get a
tax disc for your vehicle".

This change discounts the government scores by 40%. This will not affect the
ordering of government results relative to each other, just relative to
mainstream/detailed.

Ad-hoc testing of search terms which are currently returning less useful
government results over mainstream results showed that most were improved:
either completely or substantially.

Some results were not improved, such as "esa", "jsa", "maternity", but the
solution for these is probably something else; eg synonyms, content changes,
format weightings.

We were unable to find any cases where we harmed the results where the
government index contains the most relevant results. This did start to happen
if we pushed the multiplication beyond 0.6 - "mod" would stop returning the MOD
organisation page as top result at 0.5.
Scope test to top results

Reduce fragility in case results end up in the non-top-results lists in the
future.
Test that downweighted results are correctly mixed
Merge pull request #370 from alphagov/downweight_government_results

Downweight gov results to improve top 3 results