Custom perms function for shelf view

bookwyrm/models/shelf.py

@@ -1,5 +1,6 @@
 """ puttin' books on shelves """
 import re
+from django.core.exceptions import PermissionDenied
 from django.db import models
 from django.utils import timezone
 
@@ -57,6 +58,12 @@ class Shelf(OrderedCollectionMixin, BookWyrmModel):
         identifier = self.identifier or self.get_identifier()
         return f"{base_path}/books/{identifier}"
 
+    def raise_not_deletable(self, viewer):
+        """don't let anyone delete a default shelf"""
+        super().raise_not_deletable(viewer)
+        if not self.editable:
+            raise PermissionDenied()
+
     class Meta:
         """user/shelf unqiueness"""