(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/client/ui/WelcomePage.ts

@@ -98,7 +98,7 @@ export class WelcomePage extends Disposable {
           `If you already have a Grist account as `,
           dom('b', email.get()),
           ` you can just `,
-          dom('a', {href: getLoginUrl(urlState().makeUrl({}))}, 'log in'),
+          dom('a', {href: getLoginUrl()}, 'log in'),
           ` now. Otherwise, please pick a password.`
          ),
       cssSeparatedLabel('The email address you activated Grist with:'),