Convert workflow previews app to use file caching

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

mayan/apps/document_states/utils.py

@@ -0,0 +1,12 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+
+from .literals import WORKFLOW_IMAGE_CACHE_NAME
+
+
+def callback_update_workflow_image_cache_size(setting):
+    Cache = apps.get_model(app_label='file_caching', model_name='Cache')
+    cache = Cache.objects.get(name=WORKFLOW_IMAGE_CACHE_NAME)
+    cache.maximum_size = setting.value
+    cache.save()