Add users KCM

Summary:
This patch introduces a new users KCM based off of the AccountsService.

Co-authored-by: Nicolas Fella <nicolas.fella@gmx.de>

Closes T7246

Test Plan:
{F8185451}
{F8185453}
{F8185454}

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ltoscano, mart, yurchor, iasensio, meven, crossi, The-Feren-OS-Dev, davidedmundson, broulik, filipf, ngraham, nicolasfella, zzag, plasma-devel

Tags: #plasma

Maniphest Tasks: T7246

Differential Revision: https://phabricator.kde.org/D28154