groups: forward to already-joined group from GroupLink
groups: forward to already-joined group from GroupLink

fixes urbit/landscape#593

ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit