Add Cache-Control header for thumbnails

This drastically optimizes admin interface loading by telling the browser to cache thumbnails. The max-age recommendation is 1 year according to rfc2616
Closes the-paperless-project#411

src/documents/views.py

@@ -2,6 +2,7 @@
 from django.views.generic import DetailView, FormView, TemplateView
 from django_filters.rest_framework import DjangoFilterBackend
 from django.conf import settings
+from django.utils import cache
 
 from paperless.db import GnuPG
 from paperless.mixins import SessionOrBasicAuthMixin
@@ -56,10 +57,12 @@ def render_to_response(self, context, **response_kwargs):
         }
 
         if self.kwargs["kind"] == "thumb":
-            return HttpResponse(
+            response = HttpResponse(
                 self._get_raw_data(self.object.thumbnail_file),
                 content_type=content_types[Document.TYPE_PNG]
             )
+            cache.patch_cache_control(response, max_age=31536000, private=True)
+            return response
 
         response = HttpResponse(
             self._get_raw_data(self.object.source_file),