[BUGFIX] Check webmounts for backend user in workspace preview

This patch adds a check if the current backend user used for workspace
authentication has access to the current requested page. If the user
doesn't have access the workspace version of that page can't be displayed
and the live version is shown instead.

Resolves: #71734
Releases: master, 6.2
Change-Id: I66b79f9ee36ed3037729dceedd9410ccd85880f4
Reviewed-on: https://review.typo3.org/44832


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>