emails: fix exception when disabling dmail notifications.

Fix an `undefined method 'find' for Maintenance::User:Module` exception
when disabling email notifications using the "Disable notifications"
link in dmails.

test/functional/maintenance/user/email_notifications_controller.rb

@@ -0,0 +1,38 @@
+require "test_helper"
+
+module Maintenance
+  module User
+    class EmailNotificationsControllerTest < ActionDispatch::IntegrationTest
+      context "Email notifications" do
+        setup do
+          @user = create(:user, receive_email_notifications: true)
+          @verifier = ActiveSupport::MessageVerifier.new(Danbooru.config.email_key, digest: "SHA256", serializer: JSON)
+          @sig = @verifier.generate(@user.id.to_s)
+        end
+
+        context "#show" do
+          should "render" do
+            get_auth maintenance_user_email_notification_path(user_id: @user.id), @user
+            assert_response :success
+          end
+        end
+
+        context "#destroy" do
+          should "disable email notifications" do
+            delete_auth maintenance_user_email_notification_path(user_id: @user.id, sig: @sig), @user
+
+            assert_response :success
+            assert_equal(false, @user.reload.receive_email_notifications)
+          end
+
+          should "not disable email notifications when given an incorrect signature" do
+            delete_auth maintenance_user_email_notification_path(user_id: @user.id, sig: "foo"), @user
+
+            assert_response 403
+            assert_equal(true, @user.reload.receive_email_notifications)
+          end
+        end
+      end
+    end
+  end
+end