Add an option for tracking repositories of users the authenticated user follows

[jwodder]

There should be a reponews.repos.following = BOOL option (default false) for enabling tracking of repositories owned by (or affiliated with?) users that the authenticated user follows, and there should also be a [reponews.activity.following] activity preferences table for starred repositories with precedence below [reponews.activity.repo."owner/*"] and above [reponews.activity.affiliated] and [reponews.activity.starred].