Add setting migrations to apps

Add setting migrations for the common, converter, documents,
file metadata, and document signatures app.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/common/setting_migrations.py

@@ -0,0 +1,16 @@
+from __future__ import unicode_literals
+
+from mayan.apps.smart_settings.classes import NamespaceMigration
+
+from .serialization import yaml_load
+
+
+class CommonSettingMigration(NamespaceMigration):
+    """
+    From version 0001 to 0002 backend arguments are no longer quoted
+    but YAML valid too. Changed in version 3.3.
+    """
+    def common_shared_storage_arguments_0001(self, value):
+        return yaml_load(
+            stream=value or '{}',
+        )