Use srbackend=LuceneSearch on search form

To survive CirrusSearch taking over as the default search backend

This should be considered a temporary measure and removed when Cirrus
has support for 'ondiscussionpage:' (or LQT gets replaced with Flow).

Untested because I don't have Lucene/Cirrus

Change-Id: I023b8fdd7ef170d170296859a85befda95ab696f