FEAT #8338 add partial merge datasource for mailing

modules/attachments/attachments_content.php

modules/content_management/retrieve_template_from_cm.php

modules/templates/datasources/letterbox_attachment.php

src/app/attachment/controllers/AttachmentController.php

src/app/template/controllers/TemplateController.php

src/app/template/models/TemplateModelAbstract.php