Domain Models, Repositories and Validators

As Frontend User and Frontend User Group domain models have been marked as deprecated, the extension provides its implementation of these domain models and repositories for them. Names of domain models' properties have been changed a bit to make them more precise (for example, $userGroups instead of $userGroup for the property of TYPO3\CMS\Extbase\Persistence\ObjectStorage type) as well as appropriate labels in the TYPO3 backend.