Vertex Alerts

[quentinguidee]

Create a Vertex Alerts app, that can replace the notifications menu of settings. This app allows to:

• Have multiple notification destinations (Discord Webhook, WebSocket...), and there can be multiple of them;
• Have multiple notification types (instance start, stop, deleted, etc), and they can be different for each destinations;
• For the instances notifications, allow to select the instances.

Note that:

• Each app can register its own notifications and send them via the event bus:

types.EventNotification{
    Title: "...",
    Description: "...",
    Type: "...",
    // ...
}

[quentinguidee]

This should be somehow integrated with #72