Appveyor Deprecation

We are now starting to formally deprecate Appveyor in favor of Azure for builds on the win platform. Note that we have not been adding appveyor to new feedstocks for a while, so this is not a completely new change in policy. We will now, however, begin to actively disable Appveyor builds on feedstocks not using it by turning off builds for GitHub push events. Additionally, we have been issuing PRs to any remaining feedstocks to move them to Azure. We are aware that some packages built with msbuild cannot yet be moved to Azure and so are leaving Appveyor on for those feedstocks for now.