Filter top-level posts in a circle to exclude non-admins

In a circle timeline room, by default all followers are allowed to post. This is required in order to allow the followers to reply to top-level posts. Unfortunately this also means that followers can use a different app to send top-level posts into the room.

Proposed solution

Filter the top-level posts to exclude any user who does not have admin power in the room.

This doesn't stop the followers from posting into the room, but it does stop any "normal" users of the Circles app from seeing those posts. So it removes the incentive for people to do it.