OAuth 2 authentication with Azure ActiveDirectory's V2 API.

https://github.com/marknadig/omniauth-azure-oauth2 is no longer
maintained.
https://github.com/marknadig/omniauth-azure-oauth2/pull/29 contains
important additions.
This gem combines the two and makes some changes to support the full
V2 API.

The ActiveDirectory V1 auth API used OpenID Connect. If you need this,
a gem from Microsoft is available here, but seems to be abandoned.