Improve old portal timezone correction

(Still off by an hour during DST)

apiserver/import_old_portal.py

@@ -173,7 +173,7 @@ for o in old:
         new[f] = o.__dict__.get(f, None)
     new['course'] = models.Course.objects.get(id=o.course_id)
     new['old_instructor'] = o.instructor
-    new['datetime'] = str(o.datetime).replace('Z', '-07:00')
+    new['datetime'] = str(o.datetime).replace('+00:00', '-07:00')
 
     models.Session.objects.create(**new)
     print('Imported session #{} - {} {}'.format(