Channel update summary

Since this cast yesterday, we've already:
1. Brought older channel casts back to user profiles
2. Added a dedicated channels tab in notifications to everyone
3. Proposed an iteration for how replies work to be more inclusive

Still on our radar:
4. Make it easier for people to request to join a channel, leveraging the growing set of tools built by developers; channel owners can decide how they want to do it
5. Make it easier to invite people to your channel from a reply / DC / profile
6. What to do with abandoned channels