[TASK] Move FEworkspace initialization into versioning / part 2

[TASK] Move FEworkspace initialization into versioning / part 2

As there is a hook after initializing the BE user,
the code that initializes the BE user when using
a previewlink can now be moved out of the core.

Also the same hook can now be used to do the generic
workspace initialization.

Change-Id: I11ac916a02b491905cc3e22e6fdca8b934656dbd
Resolves: #30680
Releases: 4.7
Reviewed-on: http://review.typo3.org/5643
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch