Allow OAuth authentication with minimal userinfo

[acdha]

Using v5.1.5 I was trying to setup authentication using a commercial IdP which has a very limited userinfo endpoint which returns only a single value, causing Grafana to fail the login because it can't get an email address:

{
    "aud": null,
    "sub": "…",
    "unique_name": "username@example.org"
}

It'd be nice if this was at least minimally usable as would be the case with HTTP Basic authentication.

[marefr]

Duplicate of #12986