Documents: Update cache size on setting update

Add a setting callback to update the size of the document
cache when the DOCUMENTS_CACHE_MAXIMUM_SIZE setting is
updated.

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

mayan/apps/documents/utils.py

@@ -1,5 +1,16 @@
 from __future__ import unicode_literals
 
+from django.apps import apps
+
+from .literals import DOCUMENT_IMAGES_CACHE_NAME
+
+
+def callback_update_cache_size(setting):
+    Cache = apps.get_model(app_label='common', model_name='Cache')
+    cache = Cache.objects.get(name=DOCUMENT_IMAGES_CACHE_NAME)
+    cache.maximum_size = setting.value
+    cache.save()
+
 
 def parse_range(astr):
     # http://stackoverflow.com/questions/4248399/