Refactor Games, support reading Rich Presences (#877)

* Add API-level support for Rich Presences

* Add library-level support for Game presences

* Add model conversions for outgoing+incoming rich presences

* Refactored Game into Activities

* Integrated Activities with user entities

rebase hell from 5f3cb947a92f4fd01cc4df47ca548180036b47f3

* Fix JSON converters for Activities

* Finish rebase, activity should be set on BaseSocketClient

* Use ApplicationId to define a rich presence

* Added SetActivityAsync to Base and Sharded Socket clients

* Remove public parameterless Game constructor

* Remove GameAssets, refactored to GameAsset

* Hide constructors for types that should be read-only

* Revert changes to Discord.Net.sln

got damned visual studio caching

* Refactor GameParty to use dedicated current/capacity values

Per feedback from @khionu

src/Discord.Net.Core/CDN.cs

@@ -22,6 +22,12 @@ namespace Discord
         public static string GetEmojiUrl(ulong emojiId)
             => $"{DiscordConfig.CDNUrl}emojis/{emojiId}.png";
 
+        public static string GetRichAssetUrl(ulong appId, string assetId, ushort size, ImageFormat format)
+        {
+            string extension = FormatToExtension(format, "");
+            return $"{DiscordConfig.CDNUrl}app-assets/{appId}/{assetId}.{extension}?size={size}";
+        }
+
         private static string FormatToExtension(ImageFormat format, string imageId)
         {
             if (format == ImageFormat.Auto)