Add page for "estimated incoming rate" across repos

[zadjii-msft]

This came out of a discussion this week

• We know how many issues are filed a week
• We know how many get commented on

That could be turned into a heuristic for "how much incoming there is per week". That's probably a good start. A page with all your repos, and their relative incoming rates (probably broken down with a week-by-week graph and then also an "average over the last month, 6 months, year" singular stat.

The follow up to this would be to try and correlate the incoming rate with how much time is actually needed to triage that rate. Like, if we even just guess that triaging an issue takes 30 minutes on average and another comment on an issue adds another 10 minutes[1], that's even a dumb way to start estimating how much time triage costs (at a minimum).

([1]: we should use an input box for these heuristics, probably)

If we could figure out how many actually get triaged / week then we could do more magic. Then, we could correlate that rate (triaged/wk) with the incoming (incoming/wk), and knowing how much a repo is currently staffed with, figure out how much actual dev time is needed to triage all the incoming.