[BUGFIX] Ensure workspace preview works with Site Handling

While introducing Page URL handling, the workspace functionality
still built the URL with `/index.php` although the Router should
take care of that now.

As drive-by fix, the BackendUtility preview URL hook now works again
due to some refactoring back in TYPO3 9.2.

Another drive-by fix allows to register the middleware at the
right place to render the workspace preview in general.

Resolves: #86480
Releases: master
Change-Id: Icfa5a5eee304c27db39cf25903b9a14452c81168
Reviewed-on: https://review.typo3.org/58502


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>