contact-hook: no-op on accepted invite
contact-hook: no-op on accepted invite

The contact-hook was attempting to join a group upon it's invite being
accepted. However, the join poke is also sent from the frontend, causing
a potential race condition. Changes contact-hook to no-op on an
%invite-update.

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