[TASK] Cleanup BE user authentication in Frontend

The Frontend Controller (TSFE) does some initialization which
should belong to the FrontendBackendUserAuthentication directly.

Additionally, some further code cleanups are made for this area.

Resolves: #80479
Releases: master
Change-Id: I32d9c98bb511f1eaa1f56203c4678358107aa7bc
Reviewed-on: https://review.typo3.org/52177


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>