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
# Internationalization
# https://docs.djangoproject.com/en/2.2/topics/i18n/
LANGUAGE_CODE = "en-us"
TIME_ZONE = "UTC"
USE_I18N = True
USE_L10N = True
USE_TZ = True
# Static files (CSS, JavaScript, Images)
# https://docs.djangoproject.com/en/2.2/howto/static-files/
STATIC_ROOT = os.path.join(BASE_DIR, "staticfiles")
STATIC_URL = "/static/"
STATICFILES_DIRS = [
    os.path.join(BASE_DIR, "frontend/dist"),
]
STATICFILES_STORAGE = "whitenoise.storage.CompressedManifestStaticFilesStorage"
AUTH_USER_MODEL = "posthog.User"
LOGIN_URL = "/login"
LOGOUT_URL = "/logout"
LOGIN_REDIRECT_URL = "/"
APPEND_SLASH = False
CORS_URLS_REGEX = r"^/api/.*$"
REST_FRAMEWORK = {
    "DEFAULT_AUTHENTICATION_CLASSES": [
        "posthog.auth.PersonalAPIKeyAuthentication",
        "rest_framework.authentication.BasicAuthentication",
        "rest_framework.authentication.SessionAuthentication",
    ],
    "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.LimitOffsetPagination",
    "DEFAULT_PERMISSION_CLASSES": ["rest_framework.permissions.IsAuthenticated"],
    "EXCEPTION_HANDLER": "exceptions_hog.exception_handler",