"[T]he model Marketplace Notice describes affordability in terms of whether the cost of coverage exceeds 9.5% of the employee's wages from the employer, not the statutory threshold of 9.5% of the employee's household income. The model notice contains certain optional provisions on its last page that are intended to help ensure that employees understand their coverage options, but will require the notice to be customized for different groups of employees ... [T]hese optional provisions may not be practical at larger employers."