ping about forgotten reviews

[nrc]

highfive should ping reviewers who forget about a review, probably by leaving a GitHub comment and by an irc ping.

I'm not sure exactly how this should work. We could run a cron job, say every hour and do an API request to get all open PRs with assignees, then find any with no comment from the assignee in the last 24 hours. We would also have to make an API request for comments on each commit of the PR to check for comments there. We would have to check for an r+ at any time, not just in the last 24 hours in order to avoid the situation where a patch is reviewed and we're waiting for bors. We should also not ping in the case where an older PR is r+'ed and a newer one has r+ or r=foo comment from someone other than the assignee. But we should ping if there is no comment at all.

Have I missed any requirements there?