A Double Salt with Remarkable Supramolecular Channels: Synthesis and Crystal Structure of Bis[1,3-Dimesitylimidazolium] Tetrachloronickelate(Ii)-[1,3-Dimesitylimidazolium]Chloride, Which Contains Substituted Imidazolium Cations, and Both Tetrachloronickelate(Ii) and Chloride Anions