(core) Add new Grist login page

Summary:
Adds a new Grist login page to the login app, and replaces the
server-side Cognito Google Sign-In flow with Google's own OAuth flow.

Test Plan: Browser and server tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3332

app/gen-server/lib/DocWorkerMap.ts

@@ -93,6 +93,9 @@ class DummyDocWorkerMap implements IDocWorkerMap {
       },
       async close(): Promise<void> {
         _permits.clear();
+      },
+      getKeyPrefix() {
+        return formatPermitKey('', prefix);
       }
     };
     this._permitStores.set(prefix, store);
@@ -461,6 +464,9 @@ export class DocWorkerMap implements IDocWorkerMap {
       },
       async close() {
         // nothing to do
+      },
+      getKeyPrefix() {
+        return formatPermitKey('', prefix);
       }
     };
   }