## Bug description

It appears that merging people doesn't work since #10868, because the shape of a person returned by the persons endpoint turned from `{ ..., "uuid": "00000000-0000-0000-0000-000000000000", "id": 2 }` to `{ ..., "uuid": "00000000-0000-0000-0000-000000000000", "id": "00000000-0000-0000-0000-000000000000" }`. The `/merge` endpoint relies on the serial ID, and now that's not exposed (because this ID isn't even _known_ in ClickHouse).

## How to reproduce

1. Go to a random person.
2. Click "Split or merge IDs"
3. Select some other person(s) to merge into the current person
4. Notice that when you select persons, in the list they appear as some random numbers instead of distinct IDs – that's another symptom of the issue with UUIDs being returned instead of serial IDs.
  <img width="198" alt="Screen Shot 2022-07-28 at 22 24 17" src="https://user-images.githubusercontent.com/4550621/181630574-88b4c029-706a-4134-ac62-a1d0001efd3c.png">
5. Click "Merge persons"
6. See success toast, but realize that actually nothing has happened

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog (ClickHouse-based), version/commit: 1.38.0

## Additional context

This was reported by a user [on Slack](https://posthogusers.slack.com/archives/C01GLBKHKQT/p1658810378839129).

#### *Thank you* for your bug report – we love squashing them!

posthog/api/person.py
import json
from datetime import datetime
from typing import (
    Any,
    Callable,
    Dict,
    List,
    Optional,
    Tuple,
    Type,
    Union,
    cast,
)
from django.db.models import Prefetch
from drf_spectacular.types import OpenApiTypes
from drf_spectacular.utils import OpenApiExample, OpenApiParameter
from rest_framework import request, response, serializers, status, viewsets
from rest_framework.decorators import action
from rest_framework.exceptions import NotFound
from rest_framework.pagination import LimitOffsetPagination
from rest_framework.permissions import IsAuthenticated
from rest_framework.response import Response
from rest_framework.settings import api_settings
from rest_framework.utils.serializer_helpers import ReturnDict
from rest_framework_csv import renderers as csvrenderers
from statshog.defaults.django import statsd
from posthog.api.capture import capture_internal