Imports must replace drafts w/ published in split

PLAT_299

@adampalay please review & decide whether to add another bug report for old mongo (which does not work)
any(@cpennington @doctoryes @nasthagiri) please review
Perhaps more correct idea: if the draft represents a change done in dest course, then don't overwrite it. That condition is simply, `draft_dest_module.update_version != pub_dest_module.source_version`