Use old query structure to improve ranking

Copy the query structure used for the old endpoint to improve the
ranking produced.  This ranking isn't great, but is significantly better
than the pure elasticsearch ranking, and we're going to start by just
making minor tweaks to this until we have the ability to use features
based on popularity and click through as a base of our ranking.
Merge pull request #216 from alphagov/unified_ranking

Use old query structure to improve ranking

https://www.pivotaltracker.com/story/show/69916060
Bias search results away from government content

The large number of long government documents overwhelms the other
content somewhat, so multiply the scores of government documents by a
constant to reduce them.  The constant chosen here is 0.4, which is the
result of running the health check with various different constants, and
choosing the value which gave the best balance of government versus
other documents.
Merge pull request #217 from alphagov/bias_mainstream_up

Bias search results away from government content

https://www.pivotaltracker.com/story/show/69916174
Merge remote-tracking branch 'upstream/master' into make_odi_compatible