R1:8e232a8bf164

Allow Differential Mail to accept multiple comments for one mail

Summary:
Ref T2222. Currently, one `DifferentialComment` can do a lot of things (add ccs, reviewers, comments, inline comments, and perform state changes). In the future, each `ApplicationTransaction` does only one thing. This is the primary piece of complexity which makes the upcoming migration risky, because each comment needs to migrate into multiple transactions.

I want to mitigate this complexity as much as possible before the…