Switch to official SignalR package on myget

[bcanseco]

Temporarily using third-party upload of the deprecated SignalR-Server package on nuget. There should also be a nuget alpha coming very soon.

AC:

• [x] Gray.* packages removed from .csproj
• [x] No more warnings about NuGet in the Error List
• [x] Any code changes applied as necessary (might be a lot)

[bcanseco]

https://blogs.msdn.microsoft.com/webdev/2017/09/14/announcing-signalr-for-asp-net-core-2-0/