Notification: create an index for attached_to (#11050)

* Notification: cancel notifications automatically

Closes #10981

* Notifications: cancel `Project.skip` and `Organization.disabled`

Attach Django signals to `Organization` and `Project` to handle these two cases.

* Notifications: move check from each request to signal

* Notifications: use Django signals for `user_email_verified`

* Notifications: add DB index

New index for `attached_to_content_type` and `attached_to_id`. We will be making
lot of queries based on these two fields.

Related https://github.com/readthedocs/readthedocs.org/pull/11048/files#r1462286730

readthedocs/notifications/migrations/0003_notification_indexes.py

@@ -0,0 +1,23 @@
+# Generated by Django 4.2.9 on 2024-01-23 10:50
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("notifications", "0002_notification_format_values"),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name="notification",
+            options={},
+        ),
+        migrations.AddIndex(
+            model_name="notification",
+            index=models.Index(
+                fields=["attached_to_content_type", "attached_to_id"],
+                name="notificatio_attache_c6aa1d_idx",
+            ),
+        ),
+    ]

readthedocs/notifications/models.py

@@ -58,6 +58,11 @@ class Notification(TimeStampedModel):
     # notifications attached to the same object.
     objects = NotificationQuerySet.as_manager()
 
+    class Meta:
+        indexes = [
+            models.Index(fields=["attached_to_content_type", "attached_to_id"]),
+        ]
+
     def __str__(self):
         return self.message_id