Move `.github` files to the organization level
Described in https://help.github.com/en/articles/creating-a-default-community-health-file-for-your-organization, we can make a `.github` repo under pandas-dev. Then all the "community" files (code of conduct, funding, security, etc) will apply to every repo in the organization. This makes it clear that these policies apply everywhere, but we don't have to worry about copying / updating the files to every repo.