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/celery.py
from posthog.tasks.status_report import status_report
    status_report()
@app.task
def calculate_event_action_mappings():
    from posthog.tasks.calculate_action import calculate_actions_from_last_calculation
    calculate_actions_from_last_calculation()
@app.task
def calculate_cohort():
    from posthog.tasks.calculate_cohort import calculate_cohorts
    calculate_cohorts()
@app.task
def check_cached_items():
    from posthog.tasks.update_cache import update_cached_items
    update_cached_items()
@app.task
def update_cache_item_task(key: str, cache_type: str, payload: dict) -> None:
    from posthog.tasks.update_cache import update_cache_item
    update_cache_item(key, cache_type, payload)
@app.task
def send_weekly_email_report():
    pass
@app.task(bind=True)
def debug_task(self):
    print("Request: {0!r}".format(self.request))