Insurance giant UnitedHealth Group Inc. said Tuesday that it's toughening its rules on drug discounts provided via employer coverage, mandating that the savings be passed along to the end consumer. The Wall Street Journal has a report on the move by Minnetonka-based UnitedHealth (NYSE: UNH), which will affect new contracts signed beginning in 2020 for employers that use its pharmacy-benefit manager, OptumRx. Current employer plans often include discounts on various medications from drugmakers,…