[TASK] 1520 add Newsletter functionalty to sg_mail

Classes/Controller/NewsletterController.php

Classes/Domain/Repository/FrontendUserGroupRepository.php

Classes/Service/BackendService.php

Classes/ViewHelpers/Be/Menus/ActionMenuItemViewHelper.php

Resources/Private/Language/de.locallang.xlf

Resources/Private/Language/locallang.xlf

Resources/Private/Layouts/Newsletter.html

Resources/Private/Partials/Module/DocHeader.html

Resources/Private/Templates/Mail/Index.html

Resources/Private/Templates/Newsletter/Empty.html

Resources/Private/Templates/Newsletter/Index.html

Resources/Public/Contrib/select2-to-tree/select2totree.css

Resources/Public/Contrib/select2-to-tree/select2totree.js

Resources/Public/Contrib/select2/css/select2.css

Resources/Public/Contrib/select2/css/select2.min.css

Resources/Public/Contrib/select2/js/i18n/af.js

Resources/Public/Contrib/select2/js/i18n/ar.js

Resources/Public/Contrib/select2/js/i18n/az.js

Resources/Public/Contrib/select2/js/i18n/bg.js

Resources/Public/Contrib/select2/js/i18n/bn.js