Feature: CustomStatusGame Activity (#1406)

* Implement CustomStatusGame activity

Adds the CustomStatusGame class, which is the activity corresponding to the custom status feature.

* Remove unused import from Game.cs

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

@@ -5,6 +5,13 @@ namespace Discord.Rest
 {
     internal static class EntityExtensions
     {
+        public static IEmote ToIEmote(this API.Emoji model)
+        {
+            if (model.Id.HasValue)
+                return model.ToEntity();
+            return new Emoji(model.Name);
+        }
+
         public static GuildEmote ToEntity(this API.Emoji model)
             => new GuildEmote(model.Id.Value,
                 model.Name,