Separate login to a non-isolated page + Popup auth

[navzam]

Firebase login can't work in a cross-origin isolated context, so the login page needs to be served separately from the main page. The backend does not send the isolation headers for the login page.

Also change Google sign-in to use popup flow instead of redirect flow. This is required for Firefox due to partitioned storage. For details, see:

• https://developer.mozilla.org/en-US/docs/Web/Privacy/Storage_Access_Policy/Errors/CookiePartitionedForeign
• https://blog.mozilla.org/en/products/firefox/firefox-rolls-out-total-cookie-protection-by-default-to-all-users-worldwide/