R1:19572f53fdfe

Don't consider accepting on behalf of valid-but-accepted reviewers to be a validation error

Summary:
Fixes T12757. Here's a simple repro for this:

- Add a package you own as a reviewer to a revision you're reviewing.
- Open two windows, select "Accept", don't submit the form.
- Submit the form in window A.
- Submit the fomr in window B.

Previously, window B would show an error, because we considered accepting on behalf of the package invalid, as the package had already accepted.

Instead, let…