Using popups is becoming increasingly rare. Full page redirects are already used on mobile, and for some providers. This commit removes all logic related to popup authentication, leaving only the full page redirect method.
For more info, see https://meta.discourse.org/t/do-we-need-popups-for-login/127988
Regression following the ember3 upgrade. In addition to fixing, this commit consolidates our social registration logic into one place, and adds tests for the behaviour.
