[FEATURE] Improvment of the author related stuff and cleanups

Classes/Controller/NewsByAuthorController.php

Classes/Controller/SingleViewController.php

Classes/Domain/Model/Author.php

Classes/Domain/Model/Category.php

Classes/Domain/Model/News.php

Classes/Domain/Repository/NewsRepository.php

Classes/Updates/UpdateAuthors.php

Classes/UserFunction/AddAdditionalMailRecipients.php

Classes/Utility/BackendNewsUtility.php

Classes/ViewHelpers/RenderAuthorNewsViewHelper.php

Configuration/FlexForms/NewsByAuthor.xml

Configuration/TCA/Overrides/pages.php

Configuration/TCA/tx_sgnews_domain_model_author.php

Configuration/TypoScript/Frontend/constants.typoscript

Configuration/TypoScript/Frontend/setup.typoscript

README.md

Resources/Private/Language/de.locallang.xlf

Resources/Private/Language/de.locallang_db.xlf

Resources/Private/Language/locallang.xlf

Resources/Private/Language/locallang_db.xlf