Populate organization_id for AbuseReportEvent

What does this MR do and why?

In Add organization_id to abuse_report_events table (!202284 - merged) we added organization_id for abuse_report_events table. This MR starts setting this value for created abuse report events. This is second step of setting sharding key to make the table cells-compatible

The validation is behind derisk feature flag, same as !201976 (merged), just for case I missed a record creation.

It also adds validation for AntiAbuse::Reports::UserMention - we don't have any records in the table and we don't use this table yet.

All tasks of the issue

• Add organization_id column to abuse_report_even... (#563107 - closed) ✅
• Populate organization_id in abuse_report_events (#563108 - closed) (this MR)
• Backfill organization_id column in abuse_report... (#563109 - closed)
• Add sharding_key to abuse_report_events table d... (#563110 - closed)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/56311+

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Related to #563108 (closed)