Fix escaping in inbound text messages from MMG

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. We also have examples
of real inbound messages containing `👍` and `’`, so we should continue
to display these properly.

It’s a bit tricky, because the strings we get from this provider are a
mixture of escape sequences (eg `\n`) and unicode characters (eg `😨`).
So we have to first convert the unicode character `😨` into an escape
sequence, `\U0001f628` in this example. We do this by encoding with
the `raw_unicode_escape` codec:

> Latin-1 encoding with \uXXXX and \UXXXXXXXX for other code points.
> Existing backslashes are not escaped in any way. It is used in the
> Python pickle protocol.

– https://docs.python.org/3/library/codecs.html#text-encodings

Then we turn this back into a string using the `unicode_escape` codec,
which transforms all escape sequences into their literal representations
(eg `\U0001f628` becomes `😨` and `\n` becomes a newline).

app/notifications/receive_notifications.py

@@ -82,7 +82,11 @@ def receive_firetext_sms():
 
 
 def format_mmg_message(message):
-    return unquote(message.replace('+', ' '))
+    return unescape_string(unquote(message.replace('+', ' ')))
+
+
+def unescape_string(string):
+    return string.encode('raw_unicode_escape').decode('unicode_escape')
 
 
 def format_mmg_datetime(date):