Convert from project.json to VS2017 SDK style projects

[seangwright]

Motivation

project.json is deprecated and SDK style projects are supported in .NET Standard, .NET Core and .NET Framework projects.

SDK style projects also provide

• The best Nuget package dependency resolution (the transitive dependency tree can be seen in VS references UI)
• Easier access to *.csproj files in VS (no need to unload, fewer merge conflicts through file globbing)
• PackageReference Nuget package manifest syntax (the recommended approach going forward)
• Global package store for users (faster package restore for common packages)

This could make it easier for developers external to Kentico to contribute to this repository.

Proposed solution

Convert all the .csproj files in this repo to SDK project syntax

Additional context

There are tools to help with this conversion
https://github.com/hvanbakel/CsprojToVs2017
And Nate McMaster blogged about the process
https://natemcmaster.com/blog/2017/03/09/vs2015-to-vs2017-upgrade/