Backport FakeStorageSubclass from versions/next

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/storage/classes.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+
+
+class FakeStorageSubclass(object):
+    """
+    Placeholder class to allow serializing the real storage subclass to
+    support migrations.
+    """
+    def __eq__(self, other):
+        return True