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/Discord.Net.Rest.csproj

@@ -10,7 +10,8 @@
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' != 'net45' ">
-    <PackageReference Include="System.Net.Http" Version="4.3.2" /> <!-- https://github.com/dotnet/corefx/issues/19535 -->
+    <PackageReference Include="System.Net.Http" Version="4.3.2" />
+    <!-- https://github.com/dotnet/corefx/issues/19535 -->
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">
     <Reference Include="System.Net.Http" />