Nano-[Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>-R-NHMe<sub>2</sub>][H<sub>2</sub>PO<sub>4</sub>] as a Highly Effectual and Magnetically Recyclable Catalyst for the Preparation of bis(6-Amino-1,3-dimethyluracil-5-yl)methanes under Solvent-Free Conditions