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

tests/test_subversion.py

@@ -2,10 +2,10 @@
 
 """Unit test for SVN interface."""
 
-import collections
 import datetime
 import logging.config
 import unittest
+from time import sleep
 
 from bson import ObjectId
 from dateutil.tz import tzutc
@@ -391,6 +391,12 @@ class SvnTaskLoggedTest(AbstractAttractTest):
         with self.app.test_request_context():
             _, token = cli.create_svner_account('svner@example.com', self.project['url'])
 
+        # !! UGLY !!
+        # The '_created' timestamp resolution is to low. Therefor both created and commit activity can get the same
+        # created timestamp, and then the ordering of activities is non-deterministic.
+        # So lets slow things down a little.
+        sleep(0.001)
+
         # Do the push
         msg = '¡is a tie! commit to task [%s]' % shortcode
         self.post('/attract/api/%s/subversion/log' % self.project['url'],