kcm: move the settings form onto the user list's scrolling footer

The settings form lived on KCM.ScrollViewKCM's own footer, which is a fixed, non-scrolling Kirigami page footer. The form is taller than that footer is meant to hold, so on a short window (a phone-sized session, a small display) it overflows past the available height and overlaps the header instead of being reachable by scrolling.

Moves the form onto the user list's own footer instead, which scrolls with the rest of the content, so the whole form stays reachable regardless of window height. Drops extraFooterTopPadding, it only applied to the old fixed footer and has no effect on the new one.