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.
capitalisation: "posting"
capitalisation: "oauth"