Remove user association when they're no longer in an organization.

People leave organizations and GitHub organizations.

We should either remove those people, generating an event that updates the DB, or give them alumni status in some way so that the resulting view is configurable.

The issue names no files, tests, or entry points. First determine how GitHub organization membership is fetched and associations are persisted, then clarify whether removal or alumni status is intended; done should be a defined membership update with the corresponding database or event behavior and configurable view.

Written by the indexing model from the issue text.