Copy from preview to final when approving

[leplatrem]

Currently, the workflow is the following:

• changes are made on source
• review is requested: source records are copied to preview
• changes are approved: source records are copied to both preview and final

Intuitively we could change the final step and just copy from preview to final instead.

Reference https://bugzilla.mozilla.org/show_bug.cgi?id=1534644

kinto-signer/kinto_signer/listeners.py

Lines 140 to 155 in 62504c6

	if new_status == STATUS.TO_SIGN:
	# Run signature process (will set `last_reviewer` field).
	updater.destination = resource['destination']
	updater.sign_and_update_destination(event.request,
	source_attributes=new_collection,
	previous_source_status=old_status)
	if old_status == STATUS.SIGNED:
	# When we refresh the signature, it is mainly in order to make sure that
	# the latest signer certificate was used. When a preview collection
	# is configured, we also want to refresh its signature.
	if has_review_enabled:
	updater.destination = resource['preview']
	updater.sign_and_update_destination(event.request,
	source_attributes=new_collection,
	previous_source_status=old_status)