Show "using new search engine" when using Cirrus

Show CirrusSearch's now-using message on the search page when the wiki
is configured to use CirrusSearch as a primary.  This contains a link to
a help page.

Bug: 57024

Change-Id: I474f1fd8b77ada0500cb6ed842c508999ab3828d
Merge "Show "using new search engine" when using Cirrus"