FIX: Do not redirect to /auth/* urls after authentication

When using the login confirmation screen, the referrer URL is `/auth/{provider}`. That means that the user is redirected back to the confirmation screen after logging in, even though login was successful. This is very confusing. Instead, they should be redirected to the homepage.

spec/requests/omniauth_callbacks_controller_spec.rb

@@ -459,6 +459,17 @@ RSpec.describe Users::OmniauthCallbacksController do
           expect(cookie_data["destination_url"]).to eq('/t/123')
         end
 
+        it "never redirects to /auth/ origin" do
+          post "/auth/google_oauth2?origin=http://test.localhost/auth/google_oauth2"
+          get "/auth/google_oauth2/callback"
+
+          expect(response.status).to eq 302
+          expect(response.location).to eq "http://test.localhost/"
+
+          cookie_data = JSON.parse(response.cookies['authentication_data'])
+          expect(cookie_data["destination_url"]).to eq('/')
+        end
+
         it "redirects to relative origin" do
           post "/auth/google_oauth2?origin=/t/123"
           get "/auth/google_oauth2/callback"