Add Third Party sign in (Google, Github, Twitter)

[bharney]

• Add ability to sign in via Google
• Add ability to sign in via Github
• Add ability to sign in via Twitter
• On sign in click with third party provider, user is prompted for user name and password if not already logged in with that provider.
• After successful authentication, user should be required to view scope and accept or deny permissions.

I believe IdentityServer4 has this functionality baked in. If I remember correctly they redirect you to a MVC based application login before redirecting to you the main application. Wonder if there is a way to override their login screen with what you have here.