I had a hard time figuring out how to configure the correct endpoints. Maybe it helps someone.

https://github.com/jeromegn/DocumentUp/issues/23#issuecomment-4821497

Thanks to @iterateNZ and @jbbarth for reporting and helping to
troubleshoot the issue.

Laid down the foundation by using `omniauth-facebook` as a guide to the structure of this gem and the old OmniAuth determining what needs to be tested.