[TASK][DB] Move workspace preview into versioning/workspace

As the whole handling of the ADMCMD_preview is related to
version / workspaces, the according code should go in
these extensions, thus it is removed in the core.

Change-Id: Iee2f4ea42aa28825d991df92eff53c7143d93e83
Resolves: #27931
Releases: 4.6
Reviewed-on: http://review.typo3.org/3080
Reviewed-by: Stefan Neufeind
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack