Fix runtime warning when adding log entries

LogEntry.action_time expects a Django timezone object instead of a builtin datetime.

This fixes a runtime warning of the following kind:
RuntimeWarning: DateTimeField LogEntry.action_time received a naive datetime (2018-03-28 20:53:01.714173) while time zone support is active.

src/documents/signals/handlers.py

@@ -6,8 +6,7 @@
 from django.contrib.admin.models import ADDITION, LogEntry
 from django.contrib.auth.models import User
 from django.contrib.contenttypes.models import ContentType
-
-from datetime import datetime
+from django.utils import timezone
 
 from ..models import Correspondent, Document, Tag
 
@@ -107,7 +106,7 @@ def set_log_entry(sender, document=None, logging_group=None, **kwargs):
 
     LogEntry.objects.create(
         action_flag=ADDITION,
-        action_time=datetime.now(),
+        action_time=timezone.now(),
         content_type=ct,
         object_id=document.id,
         user=user,