legalpad schema

Summary:
Ref T3116. This is a large amount of schema for V0 but it seems relatively complete to the desired features in T3116.

The only thing of note that is missing is documentSignatures should have some sort of "signedStatus". "Un-signing" seemed weird to me, though I could imagination "pending signature". "Pending signature" could be done via edges pretty easily.

Plan is to have "Document" be at the top level and own policy. "DocumentBody" will store a version of title and text for each and every "edit" on a larger Document. "Edges" are to be used to tie Authors => Document for V0ish. Transactions are going to be used to store all the various edits possible here. Oh and DocumentSignatures will do what you expect, but include documentVersion as part of the key.

Test Plan: just some schema. `storage update` worked though!

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D6323