Websocket API: vlan.update not triggered on subnet_ids change

## Description
WebSocket doesn't send an update notification (vlan.update) when a new subnet is added and subsequently vlan.subnet_ids is updated.

## Steps to reproduce:
1. Go to a Subnets page
2. Open developer tools and inspect websocket connection (you may need to now refresh the page)
2. Add a new subnet

## Result:

### Actual:
no vlan.update message is sent from the server

### Expected:
vlan.update with and updated vlan.subnet_ids is sent from the server