Fix SSO login button needs to click twice to open login URL

Before this patch, the ssoUrlChanged signal might emitted before the URL actually get changed. This patch ensure the signal emitted at the correct place, and also disable the Login button before LoginHelper done its work.

To test this change, you can simply enter @a:mozilla.org (or whatever the account name is) and attempt to login it by click "continue". Then click the "Login" button and see if the first click can let your browser open the SSO login URL.