group dms: update owner when owner leaves
TODO: investigate if owners of group dms can leave them (IIRC they can, it's been a while)
though users can delete their accounts, which means that Group DMs where they are the owner get broken as the owner becomes nothing. this should be addressed.
if it is possible for the owner to be chosen, create some way to select the new owner, then do that. I'm thinking selecting based on
joined_at for that channel (so earliest member becomes owner), but we don't have that information for Group DMs.