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.Rest/Entities/Users/RestUser.cs

@@ -16,7 +16,7 @@ namespace Discord.Rest
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
         public string Discriminator => DiscriminatorValue.ToString("D4");
         public string Mention => MentionUtils.MentionUser(Id);
-        public virtual Game? Game => null;
+        public virtual IActivity Activity => null;
         public virtual UserStatus Status => UserStatus.Offline;
         public virtual bool IsWebhook => false;