Ease builds on CI and locally

[dougbu]

• react to a smallish breaking change in newer Owin packages
• gather more information when restoring RuntimePortable.sln
  • use $(RestoreUseSkipNonexistentTargets) as well
• ignore binary logs
• ensure the .NET Core 2.1 VS component is installed
  • allow use of any 2.1.5xx .NET SDK
• add a few words to code analysis dictionary
• exclude files generated for net45 legacy projects don't cause problems when compiling for netstandard2.0
• handle additional ReflectionTypeLoadExceptions when using the xUnit msbuild runner

[dougbu]

Building on TeamCity for validation

[dougbu]

This PR is sufficient for successful WSR builds on TeamCity; should be enough for successful builds on AzDO (w/ VS 2017) as well. I need to make TFS changes (related to recent Microsoft.Owin version bumps in this repo) to make TC builds entirely successful.