(core) Adding GristConnect login system

Summary:
New login system to allow simple SSO flow that is based on Discourse description that is available at:
https://meta.discourse.org/t/discourseconnect-official-single-sign-on-for-discourse-sso/13045

Test Plan: New core test.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3418

app/server/lib/DiscourseConnect.ts

@@ -31,7 +31,7 @@ const DISCOURSE_SITE = process.env.DISCOURSE_SITE;
 export const Deps = {DISCOURSE_CONNECT_SECRET, DISCOURSE_SITE};
 
 // Calculate payload signature using the given secret.
-function calcSignature(payload: string, secret: string) {
+export function calcSignature(payload: string, secret: string) {
   return crypto.createHmac('sha256', secret).update(payload).digest('hex');
 }