Moves list add notification into notification model

Right now notifications are a mix of post-save signals and clauses in
the save function of the model. I'm not actually sure which is better,
but I'm moving them to signals where it's straightforward to be
consistent.

bookwyrm/models/list.py

@@ -1,7 +1,6 @@
 """ make a list of books!! """
 import uuid
 
-from django.apps import apps
 from django.core.exceptions import PermissionDenied
 from django.db import models
 from django.db.models import Q
@@ -152,32 +151,11 @@ class ListItem(CollectionItemMixin, BookWyrmModel):
 
     def save(self, *args, **kwargs):
         """create a notification too"""
-        created = not bool(self.id)
         super().save(*args, **kwargs)
         # tick the updated date on the parent list
         self.book_list.updated_date = timezone.now()
         self.book_list.save(broadcast=False, update_fields=["updated_date"])
 
-        list_owner = self.book_list.user
-        model = apps.get_model("bookwyrm.Notification", require_ready=True)
-        # create a notification if somoene ELSE added to a local user's list
-        if created and list_owner.local and list_owner != self.user:
-            model.objects.create(
-                user=list_owner,
-                related_user=self.user,
-                related_list_item=self,
-                notification_type=model.ADD,
-            )
-
-        if self.book_list.group:
-            for membership in self.book_list.group.memberships.all():
-                if membership.user != self.user:
-                    model.objects.create(
-                        user=membership.user,
-                        related_user=self.user,
-                        related_list_item=self,
-                        notification_type=model.ADD,
-                    )
 
     def raise_not_deletable(self, viewer):
         """the associated user OR the list owner can delete"""

bookwyrm/models/notification.py

@@ -2,7 +2,8 @@
 from django.db import models, transaction
 from django.dispatch import receiver
 from .base_model import BookWyrmModel
-from . import Boost, Favorite, GroupMemberInvitation, ImportJob, Report, Status, User
+from . import Boost, Favorite, GroupMemberInvitation, ImportJob, ListItem, Report
+from . import Status, User
 
 
 class Notification(BookWyrmModel):
@@ -219,3 +220,32 @@ def notify_user_on_group_invite(sender, instance, *args, **kwargs):
         related_group=instance.group,
         notification_type=Notification.INVITE,
     )
+
+
+@receiver(models.signals.post_save, sender=ListItem)
+@transaction.atomic
+# pylint: disable=unused-argument
+def notify_user_on_list_item_add(sender, instance, created, *args, **kwargs):
+    """Someone added to your list"""
+    if not created:
+        return
+
+    list_owner = instance.book_list.user
+    # create a notification if somoene ELSE added to a local user's list
+    if list_owner.local and list_owner != instance.user:
+        Notification.notify(
+            list_owner,
+            instance.user,
+            related_list_item=instance,
+            notification_type=Notification.ADD
+        )
+
+    if instance.book_list.group:
+        for membership in instance.book_list.group.memberships.all():
+            if membership.user != instance.user:
+                Notification.notify(
+                    membership.user,
+                    instance.user,
+                    related_list_item=instance,
+                    notification_type=Notification.ADD,
+                )