Was running into an issue where because the viewer was constructed as a script viewer, it couldn't be added to the community thread with joinThread as fetchViewerIsMember always returns that script viewers are members. This diff
instead replaces the is a member check with a check for if the viewer role is positive, which will allow us to add script viewers to the thread.
Discussion at https://linear.app/comm/issue/ENG-8457/create-comm-community-for-a-farcaster-channel-if-it-doesnt-already
Depends on D13775