Using the rails `form_tag` helper generates a form with the action attribute set to the current URL (without parameters). In this case, we want to include any GET parameters, so it is better to exclude the action attribute from the form tag, and allow browsers to submit to the current URL.
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
