FEATURE: Use full page redirection for all external auth methods (#8092)

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

app/controllers/application_controller.rb

@@ -724,9 +724,6 @@ class ApplicationController < ActionController::Base
         session[:destination_url] = destination_url
         redirect_to path('/session/sso')
         return
-      elsif params[:authComplete].present?
-        redirect_to path("/login?authComplete=true")
-        return
       else
         # save original URL in a cookie (javascript redirects after login in this case)
         cookies[:destination_url] = destination_url

app/controllers/users/omniauth_callbacks_controller.rb

@@ -71,18 +71,9 @@ class Users::OmniauthCallbacksController < ApplicationController
     else
       @auth_result.authenticator_name = authenticator.name
       complete_response_data
-
-      if provider&.full_screen_login || cookies['fsl']
-        cookies.delete('fsl')
-        cookies['_bypass_cache'] = true
-        cookies[:authentication_data] = @auth_result.to_client_hash.to_json
-        redirect_to @origin
-      else
-        respond_to do |format|
-          format.html
-          format.json { render json: @auth_result.to_client_hash }
-        end
-      end
+      cookies['_bypass_cache'] = true
+      cookies[:authentication_data] = @auth_result.to_client_hash.to_json
+      redirect_to @origin
     end
   end