fix(monitor): write Proxmox's notification settings only while /etc/pve is mounted

- Nothing is written under /etc/pve unless it is the mounted cluster
  filesystem; a Monitor that starts while pve-cluster is being restarted
  waits for it and registers its webhook afterwards.
- The Monitor service is ordered after pve-cluster.service.
- The settings are written only when they have to change. The copies
  taken before a change are kept in
  /usr/local/share/proxmenux/backups/pve-notifications, and the ones left
  in /etc/pve by earlier versions are moved there.
- Uninstalling the Monitor removes its target and matcher from Proxmox.

Reported in #427.

install_proxmenux.sh

@@ -623,7 +623,7 @@ _proxmenux_rewrite_monitor_unit_for_apprun() {
         cat > "$MONITOR_SERVICE_FILE" << EOF
 [Unit]
 Description=ProxMenux Monitor - Web Dashboard
-After=network.target
+After=network.target pve-cluster.service
 
 [Service]
 Type=simple
@@ -655,7 +655,7 @@ create_monitor_service() {
         cat > "$MONITOR_SERVICE_FILE" << EOF
 [Unit]
 Description=ProxMenux Monitor - Web Dashboard
-After=network.target
+After=network.target pve-cluster.service
 
 [Service]
 Type=simple