[SECURITY] Limit user access in workspace previews

To view a preview of a workspace page a backend user is simulated.
Currently the user who created the preview link is taken into account.
This patch creates a limited backend user to be able to process the
web request.

Resolves: #28175
Releases: master, 7.6, 6.2
Security-Commit: f0445be5322b4c0e4b1c0900542aca4e00a39f46
Security-Bulletins: TYPO3-CORE-SA-2016-009, 010, 011, 012
Change-Id: I80dd9168b22bf62b2a2ed4a264240d07f056cc73
Reviewed-on: https://review.typo3.org/47610


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>