Finish migration tasks for sharding work for the fork_networks

Work covered by this issue

• Finalize BackfillOrganizationIdOnForkNetworks background migration: !188382 (merged)
• Add the NOT NULL constraint (after backfill migration is done) to move closer to sharding goal: !188272 (merged)
• Add validations in code for new column organization_id on fork_networks after backfill: !184826 (merged)

Context: this is extracted from previously created issue: #522958 (closed)