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.WebSocket/BaseSocketClient.cs

@@ -13,7 +13,7 @@ namespace Discord.WebSocket
         /// <summary> Gets the estimated round-trip latency, in milliseconds, to the gateway server. </summary>
         public abstract int Latency { get; protected set; }
         public abstract UserStatus Status { get; protected set; } 
-        public abstract Game? Game { get; protected set; }
+        public abstract IActivity Activity { get; protected set; }
 
         internal new DiscordSocketApiClient ApiClient => base.ApiClient as DiscordSocketApiClient;
 
@@ -45,6 +45,7 @@ namespace Discord.WebSocket
         public abstract Task StopAsync();
         public abstract Task SetStatusAsync(UserStatus status);
         public abstract Task SetGameAsync(string name, string streamUrl = null, StreamType streamType = StreamType.NotStreaming);
+        public abstract Task SetActivityAsync(IActivity activity);
         public abstract Task DownloadUsersAsync(IEnumerable<IGuild> guilds);  
 
         /// <inheritdoc />