R1:18938b531071

Migrate Differential comments to ApplicationTransactions

Summary:
Ref T2222. This is the big one.

This migrates each `DifferentialComment` to one or more ApplicationTransactions (action, cc, reviewers, update, comment, inlines), and makes `DifferentialComment` a double-reader for ApplicationTransactions.

The migration is pretty straightforward:

- If a comment took an action not otherwise covered, it gets an "action" transaction. This is something like "epriestley abandoned this revision.".
- If a…