Display activity for all aggregated abuse reports

What does this MR do and why?

• Resolves sub-task 4 of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/180
• We started aggregating abuse reports in !120958 (merged)
• In this MR, we will display timestamped activity for all aggregated reports

Screenshots or screen recordings

Before	After
activity-before	activity-after

How to set up and validate locally

1. In rails console

> Feature.enable(:abuse_reports_list)

# create 2 abuse reports for the same user and category
> User.limit(2).order_id_asc.pluck("id").each { |i| AbuseReport.create(reporter_id: i, user_id: User.last.id, status: "open", category: "spam", message: "test") }

1. Log-in as admin and navigate to /admin/abuse_reports. You should see text saying User X reported for spam by 2 users.
2. Click on the report. Under the Activity section, you should see two items, one for every time the user was reported.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.