Clean up sample code so that it looks less cluttered when we include the whole file as code snippet

Two things that come to mind:

1. Make the namespace simply 'Sample' for all projects
2. Reorder using statements at the top of the file

This would help improve the outcome when we include the whole source file in the snippet, as we are doing in https://github.com/aspnet/EntityFramework.Docs/pull/1422.