[TASK] Adding model & repository. Implementing index view + default layout

Classes/Controller/MailController.php

Classes/Domain/Model/RegisteredTemplate.php

Classes/Domain/Repository/RegisteredTemplateRepository.php

Configuration/TypoScript/constants.ts

Configuration/TypoScript/setup.ts

Resources/Private/Language/de.locallang.xlf

Resources/Private/Language/locallang.xlf

Resources/Private/Layouts/Default.html

Resources/Private/Templates/Mail/Index.html

ext_tables.php

ext_tables.sql