Bring back and install System Administration category

Summary:
D22742 added the info KCM to the {nav System Administration} category, but that
category isn't actually shipped with System Settings. It only appears if you have
the systemd-kcm instatted, because it ships its own local version of that file (boo).

This patch makes System Settings ship the category itself. It will be removed from
systemd-kcm in D22930.

Test Plan: {F7159603}

Reviewers: #vdg, GB_2

Reviewed By: #vdg, GB_2

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22929