(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/server/lib/Permit.ts

@@ -49,6 +49,9 @@ export interface IPermitStore {
 
   // Close down the permit store.
   close(): Promise<void>;
+
+  // Get the permit key prefix.
+  getKeyPrefix(): string;
 }
 
 export interface IPermitStores {