Redo search configuration

Allows us to enable CirrusSearch on a per-wiki basis rather than
realm-wide. Also adds new testsearch100n hosts for production and
adds LuceneSearch to $wgSearchTypeAlternatives wikis (requires
I96a08c64 from core)

Change-Id: I8b8931aace3ce7ed50a41c76405a3d800137c97d