Improved search backend fallback in Special:Search

Use the srbackend parameter if it's valid.

If it's not, use $wgSearchType if it's not null.

Finally, fall back to the search type of the database.

Bug: 53982
Change-Id: I3605cb4add270543aa20afb037401b2dfbe462f0
Merge "Improved search backend fallback in Special:Search"