Port django.utils.translation.trans_real.parse_accept_lang_header from Django 1.8

Add to dark_lang middleware

common/djangoapps/dark_lang/middleware.py

@@ -12,10 +12,12 @@ the SessionMiddleware.
 """
 from django.conf import settings
 
-from django.utils.translation.trans_real import parse_accept_lang_header
-
 from dark_lang.models import DarkLangConfig
 
+# TODO re-import this once we're on Django 1.5 or greater. [PLAT-671]
+# from django.utils.translation.trans_real import parse_accept_lang_header
+from django_locale.trans_real import parse_accept_lang_header
+
 
 def dark_parse_accept_lang_header(accept):
     '''