Add organization_id to abuse_events table

What does this MR do and why?

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

All tasks of the issue:

• Add organization_id column to abuse_events table (#561726 - closed) (this MR)
• Populate organization_id in abuse_events (#561727 - closed)
• Backfill organization_id column in abuse_events... (#561728 - closed)
• Add sharding_key to abuse_events table definition (#561729)
• Set abuse_events.organization_id as not null (#561730)

MR acceptance checklist

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Related to #561726 (closed), #553427 (closed)