Support `login_hint` in authorize request.

[evert]

When a user is redirected to the /authorize endpoint using an OpenID Connect client, we should support the login_hint query parameter here.

login_hint can be the username or email address. It's documented here https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest

The way the /authorize endpoint currently works is that it checks if a user is currently logged in, and if not it redirects the user to the /login endpoint (after login, the user goes back to /authorize).

Therefore it might make sense to support login_hint both on /authorize and /login.