Add dependencies.props

[dougbu]

• part of #228
• move to 2.1.0-preview1 packages
• switch samples from Microsoft.AspNetCore.All to Microsoft.AspNetCore
• handle IHttpRequestStreamReaderFactory namespace change

nits:

• remove UseDeveloperExceptionPage() calls since samples are all API apps
• remove Microsoft.VisualStudio.Web.CodeGeneration.Design dependencies

[ryanbrandenburg]

:shipit: after fixing build failures.

[dougbu]

69a0ad7c11