[BUG] Link to documentation instead of localhost by default (from alerts)

[Samyak2]

Describe the bug

When CHAOSGENIUS_WEBAPP_URL isn't setup correctly, all alerts (slack and email) have links ("View KPI", "Alerts dashboard", etc.) that go to localhost by default.

This can be confusing as it does not indicate how to fix it. It would be better to link to a page/section in the documentation clearly explaining that this variable isn't setup and how to set it up.

Explain the environment

• Chaos Genius version: v0.5.0 and above
• OS Version / Instance: all
• Deployment type: all

Current behavior

Links to localhost:8080

Expected behavior

Should link to documentation if not setup