fix initial loading limit of user management on large screens

[MorrisJobke]

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

[LukasReschke]

Will test on monday.

[scrutinizer-notifier]

The inspection completed: 1 new issues, 2 updated code elements

[MorrisJobke]

failing tests are unrelated

[MorrisJobke]

@LukasReschke Monday is over ;)

[ghost]

Refer to this link for build results (access rights to CI server needed):
https://ci.owncloud.org//job/pull-request-analyser-ng-simple/4572/
🚀 Test PASSed. 🚀

[LukasReschke]

@MorrisJobke Obviously I meant next monday cough cough ;-)

👍

[RobinMcCorkell]

Ah, the Tumblr problem. Code looks good 👍