[Feature] Add Support Link To Notification E-Mail Templates

Created by: Tirokk

Feature Request

Add support link to both notification e-mail templates where they are still missing:

• backend/src/middleware/helpers/email/templates/en/notification.html
• backend/src/middleware/helpers/email/templates/de/notification.html

See all other templates as examples. And see the code changes of the last PR #4787 .

Todo

• add the additional support link to the tests of the notification e-mail templates