FIX: dav_submission: proper opening metadata.txt files

After porting to our new host, an encoding error was thrown while reading metadata.txt.

dav_submission/views.py

@@ -48,7 +48,7 @@ class ListView(generic.ListView):
                 'group': None,
                 'timestamp': None,
             }
-            with open(metadata_file_path) as f:
+            with codecs.open(metadata_file_path, encoding='utf-8') as f:
                 for line in f:
                     mo = re.match(r'^Absender: (.*) <(.*)>$', line)
                     if mo is not None: