Clean up build infra to make a repository merge easier

[joelverhagen]

Just like NuGet/ServerCommon#434 but for Jobs.

I would like to try a repository merge (monorepo-ish). That is, merging NuGet Server OSS repositories into fewer (perhaps just one). To make this easier, I want to clean up and align our build infra, which is a case where the same file names exist in several repos, making a repo merge have merge conflicts.

Summary of changes

• Bring .editorconfig from NuGetGallery. This appears to be the latest and greatest.
• Update .gitignore to latest
• Delete .gitattributes, it was useless
• Update .nuget/packages.config (solution level packages) to match NuGetGallery, move to root for better visibility
• Rename SimpleVersion and SemanticVersion to JobsAssemblyVersion and JobsPackageVersion
• Fix some PowerShell functions to use allowed verbs
• Delete build.cmd, buildandtest.ps1, restoreNuGetExe.ps1 since these are not used by our CI at all.
• In build.ps1 and test.ps1 sort project specific paths and make them more DRY
• More some stuff from SdkProjects.props to Directory.Build.props
• Latest Xunit has a lot of helpful analyzers. I fixed all of the associated warnings which made the diff bigger.