Add support for relevance-ranking Ferret engine results

Summary: Ref T12819. "Relevance" here just means "how many of your search terms are present in the title?" but that's about the best we can do anyway.

Test Plan: Indexed tasks "A B", "A Z", "Z B", and "Z Z" (all with "A B" in comments). Searched for "A B". Got results ranked in the listed order, with "A B" as the most relevant hit for query "A B".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18539