Allow reviews of full stacks of pull requests

[sl]

I'm part of the beta, and it's been great using it so far. My only feedback though is: when someone else creates a stack of PRs, I want to provide review feedback across the whole stack, but the reviews are split entirely per-PR.

It would be great to be able to prepare a review across a full stack, and leave all comments at once. In particular, it would be most useful to:

• be able to mass approve a full stack
• in one sweep leave comments on everything in a stack, then at the end when I submit my review, select which parts of the stack to approve, and which to block