Review policy follow-up: What to do with PRs for which there is no reviewer

[michaelwoerister]

Updating the review policy has been discussed in #444 and a pull request to merge the new policy is open at rust-lang/rust-forge#553. One open question that come out of that PR was what to do with PRs for which no reviewer can be found.

The answer to that question is sufficiently unclear to warrant discussion on its own and finding it should not block merging the existing parts of the updated policy. I'm opening this issue so we don't forget about it. Maybe the meeting proposed in #446 will help find an answer.