Gravar-mail: Tetra­aqua­(2,2′-bipyridine-κ(2) N,N′)manganese(II) di-μ-aqua-bis­[aqua­(2,2′-bipyridine-κ(2) N,N′)(5-sulfonatoisophthalato-κO)manganate(II)] tetra­hydrate