Crash when switching from old to collaborative editor

When switching from old editor (`/w/:id`) to collaborative mode (`/w/:id/collaborate`) and adding a node, the app crashes with:

```
UndefinedFunctionError: LightningWeb.WorkflowLive.Collaborate.handle_event/3 is undefined
```

Old handler tries to send `push-change` to LiveView which doesn't exist in collaborative mode.