Add storage for new audit transactions and comments

Summary:
Ref T4896. This adds the new storage, without any code changes.

This storage is substantially identical to the Differential storage, except that `changesetID` has been replaced by `pathID`.

I've retained the properties intended to be used to implement T1460. They might not be quite right, but at least we'll be able to make any fixes consistently to both applications. For now, these fields are empty and ignored.

Test Plan: Ran `./bin/storage upgrade`. Nothing calls this code yet.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10017