Add organization_id to abuse_report_events table

What does this MR do and why?

Add organization_id column and respective index + foreign key to abuse_report_events. It also sets the table gitlab schema to gitlab_main_org.

All tasks of the issue

• Add organization_id column to abuse_report_even... (#563107 - closed) (this MR)
• Populate organization_id in abuse_report_events (#563108 - closed)
• 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 #563107 (closed)