Add debug option to down-rank closed organisations

Story at: https://www.pivotaltracker.com/story/show/66655858

This reduces the rank of closed organisations only if
`debug=downweight_closed_orgs` is added to the query parameters.  This
is to allow us to experiment with the down-ranking in production, prior
to applying it by default.

The multiplier of 0.3 was chosen, like all the other magic constants in
unified_search_builder, by trial, error, and gut feeling.

Previously, elasticsearch was combining the weights from the list of
filters it got by picking the first matching filter.  This is the
default method elasticsearch uses for `custom_filters_score`, but this
PR changes it to multiply the weights from all matching filters
together.  Previously only one of the filters would have matched, so
this shouldn't change existing rankings.
Merge pull request #295 from alphagov/downrank-closed-organisations

Add debug option to down-rank closed organisations