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
CLICKHOUSE = "clickhouse"
PRIMARY_DB = os.environ.get("PRIMARY_DB", POSTGRES)  # type: str
if PRIMARY_DB == CLICKHOUSE:
    TEST_RUNNER = os.environ.get("TEST_RUNNER", "ee.clickhouse.clickhouse_test_runner.ClickhouseTestRunner")
else:
    TEST_RUNNER = os.environ.get("TEST_RUNNER", "django.test.runner.DiscoverRunner")
if PRIMARY_DB == CLICKHOUSE:
    TEST_RUNNER = os.environ.get("TEST_RUNNER", "ee.clickhouse.clickhouse_test_runner.ClickhouseTestRunner")
else:
    TEST_RUNNER = os.environ.get("TEST_RUNNER", "django.test.runner.DiscoverRunner")
if PRIMARY_DB == CLICKHOUSE:
    TEST_RUNNER = os.environ.get("TEST_RUNNER", "ee.clickhouse.clickhouse_test_runner.ClickhouseTestRunner")
else:
    TEST_RUNNER = os.environ.get("TEST_RUNNER", "django.test.runner.DiscoverRunner")
# IP block settings
ALLOWED_IP_BLOCKS = get_list(os.environ.get("ALLOWED_IP_BLOCKS", ""))
TRUSTED_PROXIES = os.environ.get("TRUSTED_PROXIES", False)
TRUST_ALL_PROXIES = os.environ.get("TRUST_ALL_PROXIES", False)
# Quick-start development settings - unsuitable for production
# See https://docs.djangoproject.com/en/2.2/howto/deployment/checklist/
DEFAULT_SECRET_KEY = "<randomly generated secret key>"
# SECURITY WARNING: keep the secret key used in production secret!
SECRET_KEY = os.environ.get("SECRET_KEY", DEFAULT_SECRET_KEY)
ALLOWED_HOSTS = get_list(os.environ.get("ALLOWED_HOSTS", "*"))
# Metrics - StatsD
STATSD_HOST = os.environ.get("STATSD_HOST", None)
STATSD_PORT = os.environ.get("STATSD_PORT", 8125)
STATSD_PREFIX = os.environ.get("STATSD_PREFIX", None)
# Application definition
INSTALLED_APPS = [