R1:970058927987

Allow audit email to generate from multiple transactions

Summary: Ref T4896. Begins laying groundwork to split comments apart so they behave like transactions, ultimately enabling the migration.

Test Plan: Made several different types of comments, verified resulting email looks OK.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10022