mentionbot: Assign and request a review (#874)
mentionbot: Assign and request a review (#874)

This changes mentionbot to assign the PR and request a review from the
people it determines should review it instead of just posting a comment
mentioning them. These assignments will be restricted to people in the
YARPC org and any PRs with "WIP:" in the title will be skipped.


yarpc/yarpc-go,yarpc/yarpc-go