Activity preferences are not applied to renamed repos

[jwodder]

Problem: If a reponews config file uses [reponews.activity.repo.*] tables to specify activity preferences for a repository or repository owner, and that repo (owner) is later renamed on GitHub, the preferences will afterwards not be applied to the repo (owner), and the user must update the config file manually in order to restore their desired behavior. reponews should handle this case appropriately automatically.