(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/common/gutil.ts

@@ -921,3 +921,12 @@ export function getDistinctValues<T>(values: readonly T[], count: number = Infin
   }
   return distinct;
 }
+
+/**
+ * Asserts that variable `name` has a non-nullish `value`.
+ */
+export function assertIsDefined<T>(name: string, value: T): asserts value is NonNullable<T> {
+  if (value === undefined || value === null) {
+    throw new Error(`Expected '${name}' to be defined, but received ${value}`);
+  }
+}