[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: 4d89b8acd5af116444402b0fc2527fd8217e0fca
Security-Bulletins: TYPO3-CORE-SA-2016-009, 010, 011, 012
Change-Id: I4ae4b46fa3345d179cd2748ae0caa48ed3a8e4d9
Reviewed-on: https://review.typo3.org/47598


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