Electroreductive Intermolecular Coupling of Uracils with Aromatic Ketones: Synthesis of 6-Substituted and cis-5,6-Disubstituted 5,6-Dihydro-1,3-dimethyluracils and Their Transformation to 6-Substituted 1,3-Dimethyluracils, trans-5,6-Disubstituted 5,6-Dihydro-1,3-dimethyluracils, and 4,5,5-Trisubstituted 3-Methyloxazolizin-2-ones