Fix release from #1860 (hotfix #1934).

- Cron job to send weekly email report is processed multiple times. Review scheduling process to send to only one worker.
- Implement mechanism to ensure each email is sent only once to each recipient.
- Improve the way the async task is processed by creating a separate task for each organization, instead of processing all organizations in a single task.
posthog/settings.py
"""
Django settings for posthog project.
Generated by 'django-admin startproject' using Django 2.2.5.
For more information on this file, see
https://docs.djangoproject.com/en/2.2/topics/settings/
For the full list of settings and their values, see
https://docs.djangoproject.com/en/2.2/ref/settings/
"""
import ast
import os
import shutil
import sys
from distutils.util import strtobool
from typing import Dict, List, Optional, Sequence
from urllib.parse import urlparse
import dj_database_url
import sentry_sdk
from django.core.exceptions import ImproperlyConfigured
from sentry_sdk.integrations.celery import CeleryIntegration
from sentry_sdk.integrations.django import DjangoIntegration
from sentry_sdk.integrations.redis import RedisIntegration
def get_env(key):