(core) Add new Grist sign-up page

Summary:
Available at login.getgrist.com/signup, the new sign-up page
includes similar options available on the hosted Cognito sign-up
page, such as support for registering with Google. All previous
redirects to Cognito for sign-up should now redirect to the new
Grist sign-up page.

Login is still handled with the hosted Cognito login page, and there
is a link to go there from the new sign-up page.

Test Plan: Browser, project and server tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3249

stubs/app/server/lib/logins.ts

@@ -7,3 +7,7 @@ export async function getLoginSystem(): Promise<GristLoginSystem> {
   if (saml) { return saml; }
   return getMinimalLoginSystem();
 }
+
+export function getLoginSubdomain(): string | null {
+  return null;
+}