Use organization level files (e.g. CODE_OF_CONDUCT) instead of copying them to each repo

[vivienlacourba]

GitHub now has default files on the organization level for various file types such as CODE_OF_CONDUCT and CONTRIBUTING.

See: https://help.github.com/en/articles/creating-a-default-community-health-file-for-your-organization

You can add default community health files for your organization to the root of a public repository called .github that is owned by the organization.

GitHub will use and display default files for any public repository in the organization that does not have its own file of that type in any of the following places:

• the root of the repository
• the .github folder
• the docs folder

I believe we should use that feature instead of having a copy of the same CODE_OF_CONDUCT file in all repos.

This also means we should revert the change that was introduced to answer #106.