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/email.py
def __init__(
        self, subject: str, template_name: str, template_context: Optional[Dict] = None,
    ):
        if not is_email_available():
            raise exceptions.ImproperlyConfigured(
                "Email settings not configured! Set at least the EMAIL_HOST environment variable.",
            )
        self.subject = subject
        template = get_template(f"email/{template_name}.html")
        self.html_body = inline_css(template.render(template_context))
        self.txt_body = ""
        self.headers: Dict = {}
        self.to: List[str] = []
    def add_recipient(self, address: str, name: Optional[str] = None) -> None: