(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

app/client/ui/WelcomePage.ts

@@ -196,11 +196,11 @@ export class WelcomePage extends Disposable {
       'form',
       { method: "post", action: action.href },
       handleSubmitForm(pending, (result) => {
-        if (result.act === 'confirmed') {
+        if (result.status === 'confirmed') {
           const verified = new URL(window.location.href);
           verified.pathname = '/verified';
           window.location.assign(verified.href);
-        } else if (result.act === 'resent') {
+        } else if (result.status === 'resent') {
           // just to give a sense that something happened...
           window.location.reload();
         }