Stop escaping special characters in inbound

At least one of our providers gives us messages with special characters
escaped, ie a newline comes through as `\n`, not a literal newline. We
shouldn’t be showing these backslashes to any of our users.

Python has built in codecs for dealing with encoding/decoding of
strings – see
https://docs.python.org/3/library/codecs.html#text-encodings
for details. Using these builtins is safer than trying to do anything
regex or parsing-based.

app/main/views/dashboard.py

@@ -203,6 +203,9 @@ def get_inbox_partials(service_id):
             format_phone_number_human_readable(message['user_number'])
             for message in messages_to_show
         }:
+            message.update({
+                'content': bytes(message['content'], 'utf-8').decode('unicode_escape')
+            })
             messages_to_show.append(message)
 
     if not inbound_messages: