Restore search indexing to Ponder questions

Summary: Ref T3578. Get indexing back, and try to simplify it a bit.

Test Plan: Rebuilt QUES and MOCK indexes with bin/search. Created question with unique string, verified it appeared as a result. Added an answer with a unique string, got it as a result too.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3578

Differential Revision: https://secure.phabricator.com/D6619