Adds 1 to 1,000 contacts to the list and returns what happened once the import is done. New addresses become
active contacts; addresses the account already has are added to the list and, with update_existing,
get the names and contact property values sent. Every contact gets the tags. The whole request is rejected
with 400 when any contact is invalid, so nothing is imported half way. Empty contact property values are ignored;
dates are stored as days.