fix initial loading limit of user management on large screens

ref #12620 

Steps to reproduce: 
* have a large screen (vertical height) where more than 10 user rows fits in
* open user management
* before this fix: just 10 users are loaded - no lazy loading because there is no scrolling -> no invocation of lazy loading
* after this fix: the whole screen list users and there are even more - on scrolling the lazy load of additional users hits in

* requirement: many users (if you just have 10 users it won't show more ;))

cc @PVince81 @LukasReschke @blizzz