Attach PR number to logs in status handler

[bluekeyes]

https://github.com/palantir/policy-bot/blob/fec2ebf5db5c2c26a49a3675fd2ec61b46cfa6e3/server/handler/status.go#L134-L139

Because statuses match with commits, not PRs, we start out by only preparing a repository context. Once we look up the PRs, it would be good to attach the additional information (PR number) to the logger so that it is available in the evaluation logs.

While we're in here, improve the log message after PRs are found (Context event...).