models: Create MissedMessageEmailEntry table.

This will be used to store the missedmessage events received
during the waiting time for email notifications (which is currently
2 minutes, hardcoded).

The change in `test_retention` is because we've set `on_delete=CASCADE`
for the message field this table.
The new query is like so:
```
DELETE FROM "zerver_missedmessageemailentry"
    WHERE "zerver_missedmessageemailentry"."message_id" IN (
        1545, 1546, 1547, 1548, 1549, 1550, 1551, 1552, 1553
        )
```

zerver/lib/export.py

@@ -146,6 +146,7 @@ ALL_ZULIP_TABLES = {
     "zerver_scheduledemail",
     "zerver_scheduledemail_users",
     "zerver_scheduledmessage",
+    "zerver_scheduledmessagenotificationemail",
     "zerver_service",
     "zerver_stream",
     "zerver_submessage",
@@ -182,6 +183,8 @@ NON_EXPORTED_TABLES = {
     # missed-message email addresses include the server's hostname and
     # expire after a few days.
     "zerver_missedmessageemailaddress",
+    # Scheduled message notification email data is for internal use by the server.
+    "zerver_scheduledmessagenotificationemail",
     # When switching servers, clients will need to re-log in and
     # reregister for push notifications anyway.
     "zerver_pushdevicetoken",

zerver/migrations/0331_scheduledmessagenotificationemail.py

@@ -0,0 +1,58 @@
+# Generated by Django 3.2.5 on 2021-07-09 11:08
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0330_linkifier_pattern_validator"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="ScheduledMessageNotificationEmail",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True, primary_key=True, serialize=False, verbose_name="ID"
+                    ),
+                ),
+                (
+                    "trigger",
+                    models.TextField(
+                        choices=[
+                            ("private_message", "Private message"),
+                            ("mentioned", "Mention"),
+                            ("wildcard_mentioned", "Wildcard mention"),
+                            ("stream_email_notify", "Stream notifications enabled"),
+                        ]
+                    ),
+                ),
+                ("scheduled_timestamp", models.DateTimeField(db_index=True)),
+                (
+                    "mentioned_user_group",
+                    models.ForeignKey(
+                        null=True,
+                        on_delete=django.db.models.deletion.CASCADE,
+                        to="zerver.usergroup",
+                    ),
+                ),
+                (
+                    "message",
+                    models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE, to="zerver.message"
+                    ),
+                ),
+                (
+                    "user_profile",
+                    models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL
+                    ),
+                ),
+            ],
+        ),
+    ]

zerver/models.py

@@ -3340,6 +3340,33 @@ class NotificationTriggers:
     STREAM_EMAIL = "stream_email_notify"
 
 
+class ScheduledMessageNotificationEmail(models.Model):
+    """Stores planned outgoing message notification emails. They may be
+    processed earlier should Zulip choose to batch multiple messages
+    in a single email, but typically will be processed just after
+    scheduled_timestamp.
+    """
+
+    user_profile: UserProfile = models.ForeignKey(UserProfile, on_delete=CASCADE)
+    message: Message = models.ForeignKey(Message, on_delete=CASCADE)
+
+    EMAIL_NOTIFICATION_TRIGGER_CHOICES = [
+        (NotificationTriggers.PRIVATE_MESSAGE, "Private message"),
+        (NotificationTriggers.MENTION, "Mention"),
+        (NotificationTriggers.WILDCARD_MENTION, "Wildcard mention"),
+        (NotificationTriggers.STREAM_EMAIL, "Stream notifications enabled"),
+    ]
+
+    trigger: str = models.TextField(choices=EMAIL_NOTIFICATION_TRIGGER_CHOICES)
+    mentioned_user_group: Optional[UserGroup] = models.ForeignKey(
+        UserGroup, null=True, on_delete=CASCADE
+    )
+
+    # Timestamp for when the notification should be processed and sent.
+    # Calculated from the time the event was received and the batching period.
+    scheduled_timestamp: datetime.datetime = models.DateTimeField(db_index=True)
+
+
 class ScheduledMessage(models.Model):
     id: int = models.AutoField(auto_created=True, primary_key=True, verbose_name="ID")
     sender: UserProfile = models.ForeignKey(UserProfile, on_delete=CASCADE)

zerver/tests/test_retention.py

@@ -1056,7 +1056,7 @@ class TestDoDeleteMessages(ZulipTestCase):
         with queries_captured() as queries:
             do_delete_messages(realm, messages)
         self.assertFalse(Message.objects.filter(id__in=message_ids).exists())
-        self.assert_length(queries, 18)
+        self.assert_length(queries, 19)
 
         archived_messages = ArchivedMessage.objects.filter(id__in=message_ids)
         self.assertEqual(archived_messages.count(), len(message_ids))