Design fix usermgmt glitch

This fixes #9277 cc @wakeup 

While trying this, I found a showstopper though – when you horizontally scroll in the user management, the whole user list vanishes: https://github.com/owncloud/core/issues/9368

The problem is separate from this pull request though – so this can be reviewed.

Please review @Blizzz @owncloud/designers