DEV: Update confirm-email flows to use central 2fa and ember rendering (#25404)

These routes were previously rendered using Rails, and had a fairly fragile 2fa implementation in vanilla-js. This commit refactors the routes to be handled in the Ember app, removes the custom vanilla-js bundles, and leans on our centralized 2fa implementation. It also introduces a set of system specs for the behavior.

lib/second_factor/actions/confirm_email.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module SecondFactor::Actions
+  class ConfirmEmail < Base
+    def no_second_factors_enabled!(params)
+      # handled in controller
+    end
+
+    def second_factor_auth_required!(params)
+      {
+        callback_params: {
+          token: params[:token],
+        },
+        redirect_url:
+          (
+            if @current_user
+              "#{Discourse.base_path}/my/preferences/account"
+            else
+              "#{Discourse.base_path}/login"
+            end
+          ),
+      }
+    end
+
+    def second_factor_auth_completed!(callback_params)
+      # handled in controller
+    end
+  end
+end