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/test/test_email.py
with self.settings(EMAIL_HOST=None):
            with self.assertRaises(ImproperlyConfigured) as e:
                EmailMessage("Subject", "template")
            self.assertEqual(
                str(e.exception), "Email settings not configured! Set at least the EMAIL_HOST environment variable.",
            )
    @freeze_time("2020-09-21")
    def test_weekly_email_report(self) -> None: