Refactor GameParty to use dedicated current/capacity values

Per feedback from @khionu

src/Discord.Net.Core/Entities/Activities/GameParty.cs

@@ -5,6 +5,7 @@
         internal GameParty() { }
 
         public string Id { get; internal set; }
-        public int[] Size { get; internal set; }
+        public int Members { get; internal set; }
+        public int Capacity { get; internal set; }
     }
 }

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -56,10 +56,18 @@
 
         public static GameParty ToEntity(this API.GameParty model)
         {
+            // Discord will probably send bad data since they don't validate anything
+            int current = 0, cap = 0;
+            if (model.Size.Length == 2)
+            {
+                current = model.Size[0];
+                cap = model.Size[1];
+            }
             return new GameParty
             {
                 Id = model.Id,
-                Size = model.Size
+                Members = current,
+                Capacity = cap,
             };
         }