Add SSO (OpenID) authentication

[rolestack]

It would be great if SSO login were available!
Instead of manually entering a TOTP code every time for security, logging in via SSO would be much more convenient🚀🚀🚀

[BobWs]

PocketID support would be nice for SSO implementation!

[ablyes]

Great idea. I'm also tired of double login in authentik and flatnotes which i use a lot.

[anujc4]

So I took a crack at this in #351.

If anyone wants to test it out, replace the docker image with anujchandra/flatnotes:oidc and integrate it with your OIDC provider. I tested with PocketID and it works, happy to take feedback.