Extend "fulltext" and "ngrams" interfaces from "indexable" interface

Summary: Ref T8788. See D17702. This allows bin/search index to index stuff which only implements Ngrams, not Fulltext.

Test Plan: Kinda poked around bin/search index a bit, yell if you hit more issues deeper down the stack?

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T8788

Differential Revision: https://secure.phabricator.com/D17704