Support using the Porter stemmer when compiling search queries

Summary:
Ref T6740. Stemming reduces words to roots, so "deleting", "deletion", and "delete" all index as the root "delet". It's fine that roots aren't really words.

When users search for "delete", we'll search for "delet" intead, in an index which has already indexed all variants of "delete" as "delet".

(This gets a bit tricky when users mix quoted and unquoted terms, so we may need to search literal and stemmed indexes separately.)

Test Plan: Added unit tests; ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6740

Differential Revision: https://secure.phabricator.com/D16943