Add support for social authentication

[davidfowl]

This change adds support for creating users from social accounts. Those accounts are registered with the backend API in exchange for a JWT token. That token can then be used to make further API calls.

Configuration for these providers must be added in configuration with the following schema:

{
    "Authentication": {
        "Schemes": {
            "<scheme>": {
                "ClientId": "xxx",
                "ClientSecret": "xxxx"
            }
        }
    }
}

The client id and client secret must be specified, or it will fail. This support is built on top of ASP.NET Core's OAuthAuthenticationHandler. Other providers can be found here https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers#providers