R1:6052bc193344

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