Normalize issue counts by the rate at which repositories get new issues

[jyasskin]

It's not really fair that csswg-drafts winds up at the top of https://jyasskin.github.io/spec-maintenance/. I can cheaply ask Github how many new issues and PRs were filed in a repository over the last 6 months, and divide the out-of-SLO count by that before sorting.

[svgeesus]

On the one hand it is fair, because there is certainly a large pile of still-open issues.

On the other hand, most WGs aren't maintaining 140 specifications (and that is without counting Houdini and FX TF specifications).