[FEATURE] Use FlashMessage in be_user module

Add FlashMessages:
* info about cached grouplists being updated
* when there are too many users to show

Change-Id: Iedd62f7731e61a6dff7ca9b6d477d75524351828
Resolves: #28543
Reviewed-on: http://review.typo3.org/3817
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert
Reviewed-by: Stefan Neufeind
Reviewed-by: Georg Ringer