Metal‐Ion‐Templated Polymers: Synthesis and Structure of N‐(4‐Vinylbenzyl)‐1,4,7‐Triazacyclononanezinc( II ) Complexes, Their Copolymerization with Divinylbenzene, and Metal‐Ion Selectivity Studies of the Demetalated Resins—Evidence for a Sandwich Complex in the Polymer Matrix