Simulate Frontend User Group v12

[leitgab]

12.x-branch still uses the hook $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['tslib/class.tslib_fe.php']['hook_checkEnableFields']['direct_mail'] to implement the simulate frontend user group function.

Said hook was removed with TYPO3 12 and event listeners have to be used instead:
https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.0/Breaking-96996-HookCheckEnableFieldsRemoved.html

The result is, that the simulate frontend user group function does not work since TYPO3 12 any more.