Backport FakeStorageSubclass from versions/next

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

mayan/apps/documents/migrations/0048_auto_20190711_0544.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.22 on 2019-07-11 05:44
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import mayan.apps.documents.models.document_version_models
+import mayan.apps.storage.classes
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0047_auto_20180917_0737'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='documentversion',
+            name='file',
+            field=models.FileField(storage=mayan.apps.storage.classes.FakeStorageSubclass(), upload_to=mayan.apps.documents.models.document_version_models.UUID_FUNCTION, verbose_name='File'),
+        ),
+    ]

mayan/apps/documents/storages.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals
 
-from django.utils.module_loading import import_string
+from mayan.apps.storage.utils import get_storage_subclass
 
 from .settings import (
     setting_documentimagecache_storage,
@@ -8,10 +8,10 @@ from .settings import (
     setting_storage_backend, setting_storage_backend_arguments
 )
 
-storage_documentversion = import_string(
+storage_documentversion = get_storage_subclass(
     dotted_path=setting_storage_backend.value
 )(**setting_storage_backend_arguments.value)
 
-storage_documentimagecache = import_string(
+storage_documentimagecache = get_storage_subclass(
     dotted_path=setting_documentimagecache_storage.value
 )(**setting_documentimagecache_storage_arguments.value)