[BUGFIX] Fix Online BE-Users->Terminate be user sessions termination

Wrong view helper notation lead to it not being rendered.
Additionally flash message enqueue is used with a string instead
of a flash message object, which lead to a PHP fatal error.

* Fix the vh notation
* Use addFlashMessage() to add the flash message
* Make the message translatable

Resolves: #63439
Releases: master
Change-Id: Iddbc39ba0fd55d2ee0ac8610e9c8b8b96b614633
Reviewed-on: http://review.typo3.org/34799


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>