(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

package.json

@@ -66,6 +66,7 @@
     "mocha-webdriver": "0.2.9",
     "moment-locales-webpack-plugin": "^1.2.0",
     "nodemon": "^2.0.4",
+    "otplib": "12.0.1",
     "selenium-webdriver": "3.6.0",
     "source-map-loader": "^0.2.4",
     "stats-webpack-plugin": "^0.7.0",