chore(Persons): Remove foreign key constraints for all persons models

[timgl]

Problem

We want to move persons and related models into its own DB. In order to do that, we need to remove foreign key constraints.

In the below set of models (which we'll move to the new DB), change all foreign keys that don't reference each other (just team_ids and cohort for cohortpeople) into ints, so that we can only move these tables over (or vice versa)
Person
PersonDistinctId
PersonlessDistinctId
PersonOverrideMapping
PersonOverride
PendingPersonOverride
FlatPersonOverride
FeatureFlagHashKeyOverride
CohortPeople

Also remove persons from admin, just to avoid another write path/django doing funky things without foreign keys

Changes

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

How did you test this code?

[greptile-apps]

PR Summary

This migration removes foreign key constraints from person-related tables to prepare for moving them to a separate database, focusing on decoupling these models from the main PostHog database.

• Drops team_id and is_user_id foreign key constraints from posthog_person table
• Removes team_id foreign key constraints from posthog_persondistinctid and posthog_personlessdistinctid tables
• Removes team_id constraint from posthog_featureflaghashkeyoverride table
• Drops cohort_id foreign key constraint from posthog_cohortpeople table

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[greptile-apps]

logic: Empty state_operations means Django's ORM will still expect foreign key relationships. Ensure application code is updated accordingly.