Move some value massaging into SearchFields

Summary: Ref T8441. Ref T7715. Let SearchFields do some value massaging before buiding queries so we don't have to work as hard in each SearchEngine. Particularly, they can handle evaluateTokens() from Tokenizers.

Test Plan: Paste automatically gained access to viewer(), etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7715, T8441

Differential Revision: https://secure.phabricator.com/D13170