[TASK] TSFE - Add hook after BE_USER init to clean up workspace functionality

In order to move workspace logic out of the TYPO3 core and into
versioning/workspaces extensions, a new hook is required first.
The hook will be placed after the backend authorization, and in
a followup issue the hook will be used for further cleanups of the
TSFE issue.

Change-Id: Ica08ce0c9d3802c6e9cd1b97d91f3d7a4efebd0f
Resolves: #30646
Releases: 4.6
Reviewed-on: http://review.typo3.org/5631
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers