[FEATURE] Add Mail Layouts

Classes/Controller/LayoutController.php

Classes/Controller/MailController.php

Classes/Domain/Model/Layout.php

Classes/Domain/Model/Template.php

Classes/Domain/Repository/LayoutRepository.php

Classes/Form/NodeExpansion/FieldInformation.php

Classes/Hooks/ProcessDatamap.php

Classes/Service/BackendService.php

Classes/Service/MailTemplateService.php

Classes/ViewHelpers/Backend/ControlViewHelper.php

Configuration/TCA/tx_sgmail_domain_model_layout.php

Configuration/TCA/tx_sgmail_domain_model_template.php

Resources/Private/Language/de.locallang.xlf

Resources/Private/Language/de.locallang_db.xlf

Resources/Private/Language/locallang.xlf

Resources/Private/Language/locallang_db.xlf

Resources/Private/Partials/Layout/Empty.html

Resources/Private/Partials/Module/DocHeader.html

Resources/Private/Templates/Layout/Index.html

Resources/Private/Templates/Mail/Index.html