Free up membership seats

This is a tracking issue to systematically free up membership seats. We're running into issues where we can't invite members to private repos because we don't have membership seat.

We ideally want to have a way to identify users that have had no contribution (PR/issue/discussion creation/comment) in the last X months/years.

Another low hanging fruit area is anyone who is a `members.member` but isn't part of a repo or team.

We want a way to still acknowledge their past contribution / presence in the organization. An Alumni team is one idea.

Another option is to make these folks outside collaborators, but I don't believe github-mgmt supports this functionality right now. It would be ideal if github-mgmt listed our outside collaborators.

There has been efforts in the past to cleanup the roster list. We should see what we can draw from in the past and also devise something easily repeatable for the future for when we next run into this.