Automatically share forum with contact if user and contact have both posted
If the user and a contact both subscribe to a given forum and have made their subscriptions known to each other by posting to the forum, we could automatically share the forum with the contact. This would make the forum's sync graph more robust, and would save some effort when sharing a forum among a group of contacts.
We'd want some onboarding the first time this happened, to explain that the forum would be shared automatically and to guide the user to the sharing status screen.