Resolve "Persist Prometheus alert events"

What does this MR do?

Create a table to store alert events arriving in Projects::Prometheus::AlertsController#notify action.

Note, this MR will based on changes done in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7382.

What are the relevant issue numbers?

#7756 (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 #7756 (closed)