Move LegalpadDocument to new phid stuff

Summary: ref T2715 - apologies btw as I didn't catch the "start from the bottom" ask until recently... :/

Test Plan: phid.query for some legalpad documents... booya. also loaded legalpad.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2715

Differential Revision: https://secure.phabricator.com/D6576