Make the deletion check a default filter on the base model, inherited by
subclasses of the base model.
diff --git a/planetstack/core/models/plcorebase.py b/planetstack/core/models/plcorebase.py
index 9ce409d..2f7fd82 100644
--- a/planetstack/core/models/plcorebase.py
+++ b/planetstack/core/models/plcorebase.py
@@ -18,6 +18,8 @@
     def notify_observer(*args, **kwargs):
         pass
 
+# This manager will be inherited by all subclasses because
+# the core model is abstract.
 class PlCoreBaseManager(models.Manager):
     def get_query_set(self):
         return super(PlCoreBaseManager, self).get_query_set().filter(deleted=False)
@@ -34,6 +36,8 @@
     deleted = models.BooleanField(default=False)
 
     class Meta:
+        # Changing abstract to False would require the managers of subclasses of
+        # PlCoreBase to be customized individually.
         abstract = True
         app_label = "core"