Disable systemd services when stopping to uninstall them

Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).

We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.

roles/matrix-bridge-appservice-slack/tasks/setup_uninstall.yml

@@ -9,6 +9,7 @@
   service:
     name: matrix-appservice-slack
     state: stopped
+    enabled: no
     daemon_reload: yes
   when: "matrix_appservice_slack_service_stat.stat.exists"