Check the server is up

Pull request feedback: a review comment, a review that requests changes, a comment that mentions `@pistis`, or a failing check on a pull request a session opened becomes a message in that session, as the person who wrote it and only if they may message it. Point the code host's webhook here (GitHub: the App's webhook URL) with the connection's webhook secret; the code host signs every delivery. Anything not acted on is recorded with why.