User Manager KCM text changes and UI modernization

Summary:
BUG: 386635

Improve some strings and use a more modern checkboxes-on-the-left-side design

Test Plan:
Tested in KDE Neon:
{F5486134}

Reviewers: abetts, #vdg, #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8755