Incorrect alert sending logic means that some failed alerts never get re-tried

[evansd]

The send_monthly_alerts command contains logic to avoid re-sending alerts which have already been sent. However this works by checking whether the user has been send any alerts for the current month, rather than whether they've been sent an alert for the specific bookmark:

openprescribing/openprescribing/frontend/management/commands/send_monthly_alerts.py

Lines 108 to 111 in b9f2afe

	query = (
	Q(user__is_active=True)
	& ~Q(user__emailmessage__tags__contains=["measures", now_month])
	&

This means that if some, but not all, of a user's alerts get sent successfully then subsequent runs of the command will never try to resend the failed ones.

There's no way to fix this without a db migration because the current EmailMessage object has no link back to its source bookmark object:

openprescribing/openprescribing/frontend/models.py

Lines 999 to 1007 in b9f2afe

	class EmailMessage(models.Model):
	message_id = models.CharField(max_length=998, primary_key=True)
	pickled_message = models.BinaryField()
	to = ArrayField(models.CharField(max_length=254, db_index=True))
	subject = models.CharField(max_length=200)
	tags = ArrayField(models.CharField(max_length=100, db_index=True), null=True)
	created_at = models.DateTimeField(auto_now_add=True, db_index=True)
	user = models.ForeignKey(User, null=True, blank=True, on_delete=models.CASCADE)
	send_count = models.SmallIntegerField(default=0)