"fix" github scope-upgrade flow

Summary:
Can't say I know what I'm doing here, but this fixes an the upgrade-scope flow for landing-to-github.

Without this change, it looks like the submit button makes the browser (Chrome and msie) make the call in the background, instead of hijacking the window.
With it, it works like it should.

Test Plan: try to land with weak token, click "Refresh..", see GitHub button.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9407