Add organization_id to abuse_reports table

What does this MR do and why?

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

All tasks of the issue:

• Add organization_id column to abuse_reports table (#561343 - closed) (This MR)
• Populate organization_id in abuse_report (#561344 - closed)
• Backfill organization_id column in abuse_report... (#561345 - closed)
• Add sharding_key to abuse_reports table definition (#561347 - closed)
• Set abuse_reports.organization_id as not null (#561466)

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Closes #561343 (closed)