Handle FxA sign in from email (bug 1080868)

https://bugzilla.mozilla.org/show_bug.cgi?id=1080868

Reuse the window that was used for FxA sign in if there is no `window.opener`. This enables sign in when the user navigates to FxA from one of our pre-verified emails.

See also https://github.com/mozilla/zamboni/pull/2652.
This URL is included in the OAuth provider config, for example: https://github.com/mozilla/fxa-dev/blob/master/roles/oauth/templates/config.json.j2

There have been discussions of changing what's presented to the user upon email confirmation, but currently it does show that url.