(core) add a login method based on headers

Summary:
This fleshes out header-based authentication a little more to
work with traefik-forward-auth.

Test Plan: manually tested

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3348

stubs/app/server/lib/logins.ts

@@ -1,9 +1,10 @@
+import { getForwardAuthLoginSystem } from 'app/server/lib/ForwardAuthLogin';
 import { GristLoginSystem } from 'app/server/lib/GristServer';
 import { getMinimalLoginSystem } from 'app/server/lib/MinimalLogin';
 import { getSamlLoginSystem } from 'app/server/lib/SamlConfig';
 
 export async function getLoginSystem(): Promise<GristLoginSystem> {
-  const saml = await getSamlLoginSystem();
-  if (saml) { return saml; }
-  return getMinimalLoginSystem();
+  return await getSamlLoginSystem() ||
+    await getForwardAuthLoginSystem() ||
+    await getMinimalLoginSystem();
 }