Support for Generic OpenID Connect Identity Providers

@tommoor I see you're currently working on authentication, specifically #1945. I would just suggest adding support for OAuth2 and OpenID Connect in general would increase Outline's ability to be integrated into existing infrastructure as these are the current popular standards. It would allow you to specify and control what the returned identity token (JWT) would need to look like and how the claims are mapped to your internal attributes, thus allowing others to integrate with your code, rather than you having to create potentially hundreds of integrations with others. I would mention also that a [drop-in module for Passport.JS exists for this]( This wouldn't remove your need to create direct integrations with other IdPs like those of Google, MS, et al. but it would certainly allow the community to meet you half way. Those of us who care would also be able to integrate additional specialized identity providers like those mentioned above and even other protocols s…