R1:9a1d59ad5bd5

Separate sever-side typeahead queries into "prefix" and "content" phases

Summary:
Ref T8510. When users type "platypus" into a typeahead, they want "Platypus Playground" to be a higher-ranked match than "AAA Platypus", even though the latter is alphabetically first.

Specifically, the rule is: results which match the query as a prefix of the result text should rank above results which do not.

I believe we now always get this right on the client side. However, WMF has at least one case (described in…