Add a section about triage statistics in the quality notification

For example, # of S1, # of bugs with no priority, time to triage, etc., etc..

We could start with average time to set priority/severity, give a list of bugs which haven't had a priority/severity set for a long time, and give a list of bugs with a needinfo pending for a long time.

For now I won't do this because it risks making the notification too long, I will start with #2534 instead which basically covers what we care about most (regressions having priority and severity set so that release managers know what to expect).