Resolve "Listen for resolved Prometheus alerts"

What does this MR do?

Start listening to resolved Prometheus alerts. Before this MR Prometheus sent only firing alerts.

In order to implement Operations Homepage MVC we need to track alerts' status to show on the very Operations Homepage.

The first step is to let Prometheus send also resolved alerts.

Persisting alert events and sending notification mails will be done in separate MRs to keep the scope of this MR narrow.

What are the relevant issue numbers?

#7495 (closed) #5781 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7495 (closed)