Reduce stale management VLAN entries on config save

The firmware has a single management VLAN (one management_vlan variable),
so a stored config should never carry more than one "vlan N mgmt" line.
When parsing a mgmt command, drop any previously stored mgmt entry so
repeatedly changing the management VLAN no longer accumulates stale lines.

html/config.js

@@ -77,6 +77,9 @@ function parseConf(s){
         break;
       }
     }
+    // Only one management VLAN can be active, so drop any previous mgmt entry
+    if (/^vlan\s+\d{1,4}\s+mgmt$/.test(line))
+      configuration = configuration.filter(item => !/^vlan\s+\d{1,4}\s+mgmt$/.test(item));
     configuration.push(line);
   }
   console.log("Configuration now:");