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_beta.sh

@@ -496,7 +496,7 @@ update_monitor_service() {
     cat > "$MONITOR_SERVICE_FILE" << EOF
 [Unit]
 Description=ProxMenux Monitor - Web Dashboard (Beta)
-After=network.target
+After=network.target pve-cluster.service
 Before=shutdown.target reboot.target halt.target
 Conflicts=shutdown.target reboot.target halt.target
 
@@ -535,7 +535,7 @@ create_monitor_service() {
         cat > "$MONITOR_SERVICE_FILE" << EOF
 [Unit]
 Description=ProxMenux Monitor - Web Dashboard (Beta)
-After=network.target
+After=network.target pve-cluster.service
 Before=shutdown.target reboot.target halt.target
 Conflicts=shutdown.target reboot.target halt.target