Lightly modernize LegalpadDocumentSearchEngine

Summary: Depends on D18785. Ref T13024. While I'm in here, update this a bit to use the newer stuff.

Test Plan: Searched for Legalpad documents, saw more modern support (subscribers, order by, viewer()).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13024

Differential Revision: https://secure.phabricator.com/D18786