R1:a69cca9fbb15

Update overall revision status after reviewers change

Summary:
Ref T2222. This doesn't feel super clean, but doesn't feel too bad either.

Basically, Differential transactions can have secondary state-based effects (changing the overall revision status) when reviewers resign, are removed, accept, or reject revisions.

To deal with this in ApplicationTransactions, I did this:

- `applyFinalEffects()` can now alter the transaction set (notably, add new ones). This mostly matters for email, notifications…