[TASK] Decouple adminPanel from frontend

The admin panel has been extracted into an own extension. To enable
users to de-activate the admin panel completely, the hard coupling
between the extension and other parts of the core had to be resolved.

With this change, the initialization of both adminPanel and feedit
were moved into PSR-15 middlewares. Additionally all parameters
related to the adminPanel were removed from the FrontendBackend-
UserAuthentication.

As feedit is tigthly coupled with the adminPanel some changes had
to be made to its initialization, too.

The flow of the adminPanel initialization and rendering were
streamlined to allow modules to make use of the request object.

Due to these changes in the control flow of the application the
two existing tests were removed and new tests will be rewritten
once the API is declared as stable.

Releases: master
Resolves: #84641
Change-Id: I72beefde0d792d3f4295c45aa27204c817d2de7a
Reviewed-on: https://review.typo3.org/56558
Tested-by: TYPO3com <no-repl...