Detect r? without @

[nrc]

We currently look for r? @foo in messages, but people often write r? foo. We should check for this and assign a reviewer for it (ignoring r? without a reviewer which is also common).

This will get some false positives, e.g., r? anyone, but I think that is OK, the attempt to set the assignee on GitHub will silently fail. In that case, we should then set a random reviewer (we should close this loophole in any case, I think).