Move Prometheus alerts from EE to CE

Summary

Contributes to #42640 (closed)

Move Integrations -> Prometheus -> Alerts from EE domain to CE.

Dependencies:

• Feature spec
• Route
• _external_alerts.html.haml partial
• Vue component
• projects_with_prometheus_alerts metric from usage_data.rb

Improvements

Moving Prometheus alerts from EE to CE requires a lot of code changes. All the required changes can be split and moved in several steps (as described here).

Risks

Involved components

Optional: Intended side effects

Optional: Missing test coverage