Don't send cookbook notifications to imported users

:fork_and_knife: Since we'll be running Supermarket in parallel with the current community site we don't want to send cookbook follower notifications from Supermarket until the user signs in, at least during the soft launch period. Imported users will have a `oauth_token` for their `chef_oauth2` account set to imported until they sign in so we can use that as an indicator for whether or not they are active on Supermarket.