Add banner (large) image

PluralKit.Core/Models/PKSystem.cs

@@ -14,6 +14,7 @@ namespace PluralKit.Core {
         public string Description { get; }
         public string Tag { get; }
         public string AvatarUrl { get; }
+        public string BannerImage { get; }
         public string Color { get; }
         public string Token { get; }
         public Instant Created { get; }