Emit $wgSearchType as JavaScript config variable

Allows for the search interface to be customzied for the particular
search-engine used, and allows collecting client-side performance measurements
that specify which search engine was used.

Change-Id: Ibeda834e9d5dbaf1d7e40c2dacbc60feb2cc4bba
(cherry picked from commit ac99ef7c25a312198dad85c006cc8f7b3acfaf6f)