Introduce new global config to disable user enumeration - first to be us...

...e in the sharing drop down - more to follow ???

@karlitschek to be used in certain education setups

@schiesbn @PVince81 @LukasReschke  please review - THX
I wonder why you still perform the whole search just to remove all results at the end again beside the exact name typed by the user? Wouldn't it make sense to skip the whole search in this case and just send the share request for the given user once the user hit "enter"?