Notifications regression: Notifications not created

Notifications for when someone posted a comment on your node
was not created.

Root cause was that default values defined in schema was not set,
resulting in activity subscriptions not being active.
There were 2 bugs preventing them to be set:
* The way the caching of markdown as html was implemented caused
  default values not to be set.
* Eve/Cerberus regression causes nested default values to fail
  https://github.com/pyeve/eve/issues/1174

Also, a 3rd bug caused nodes without a parent not to have a
subscription.

Migration scripts:
How markdown fields is cached has changed, and unused properties
of attachments has been removed.
./manage.py maintenance replace_pillar_node_type_schemas

Set the default values of activities-subscription
./manage.py maintenance fix_missing_activities_subscription_defaults

pillarsdk/utils.py

@@ -178,10 +178,16 @@ def remove_private_keys(document):
     dictionary.
     """
 
+    def do_remove(doc):
+        for key in list(doc.keys()):
+            if key.startswith('_'):
+                del doc[key]
+            elif isinstance(doc[key], dict):
+                doc[key] = do_remove(doc[key])
+        return doc
+
     doc_copy = copy.deepcopy(document)
-    for key in list(doc_copy.keys()):
-        if key.startswith('_'):
-            del doc_copy[key]
+    do_remove(doc_copy)
 
     try:
         del doc_copy['allowed_methods']