Update Horizon log in page to the new design

Update Horizon log-in page to the new design.

Note: the "forgot password" won't be implemented at this point, because the backend doesn't support it yet and it seems not practical to add it to the existing system. I gues eventually we'll need to use OAuth 2.0, then all the authentication stuff will be moved to the auth server, then the "forgot password" will be automatically handled there.