Upgrade to ASP.NET Core 2.1

[stevejgordon]

I've completed an initial test in a local branch against 2.1 preview 2 which seems to work.

@MisterJames I presume we want to wait until RC / RTM before we take this step? If we do it early we can start to use the new shiny but contributors would need VS 2017 15.7 preview 3 + the preview 2 SDK etc so we add a barrier of entry.