Use transactions to apply "add auditors" action in Audit

Summary:
Ref T4896. Move the write for "Add Auditors" inside the new Editor.

There are no longer any readers or writers for metadata, so remove the calls for it.

Test Plan: Added auditors from the web UI.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10123