LP#1549505: add flag to tweak popularity-adjusted relevance

LP#1549505: add flag to tweak popularity-adjusted relevance

This adds a new global_flag, search.max_popularity_importance_multiplier,
to control the factor by which popularity affects Popularity Adjusted
relevance ranking.

The value should be a decimal number, typically between 1.0 and 2.0:

* 1.0 be would be equivalent to not adjusting relevance for popularity
  at all.
* 1.1 would mean that the multiplier would range from 1 (for zero
  popularity) to 1.1 (for maximum popularity)
* values less than 1.0 would have the effect of boosting lower
  popularity records in search results -- dramatically

Signed-off-by: Galen Charlton <gmc@esilibrary.com>