Use red/green indicator for discord & slack webhooks#1405

The current webhook icon is the green UptimeRobot logo, which is confusing in a glance when looking at a “Monitor is DOWN” notification.

I suggest one of the following:

1. Change the icon depending on status (red for DOWN)
2. Using the embed format so you can color it, or
3. Adding an emoji at the beginning of the first line like: “🔴 Monitor is DOWN” or “✅ Monitor is UP”