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).

tests/app/notifications/test_receive_notification.py

@@ -12,6 +12,7 @@ from app.notifications.receive_notifications import (
     create_inbound_sms_object,
     strip_leading_forty_four,
     has_inbound_sms_permissions,
+    unescape_string,
 )
 
 from app.models import InboundSms, EMAIL_TYPE, SMS_TYPE, INBOUND_SMS_TYPE
@@ -166,6 +167,36 @@ def test_format_mmg_message(message, expected_output):
     assert format_mmg_message(message) == expected_output
 
 
+@pytest.mark.parametrize('raw, expected', [
+    (
+        '😬',
+        '😬',
+    ),
+    (
+        '1\\n2',
+        '1\n2',
+    ),
+    (
+        '\\\'"\\\'',
+        '\'"\'',
+    ),
+    (
+        """
+
+        """,
+        """
+
+        """,
+    ),
+    (
+        '\x79 \\x79 \\\\x79',  # we should never see the middle one
+        'y y \\x79',
+    ),
+])
+def test_unescape_string(raw, expected):
+    assert unescape_string(raw) == expected
+
+
 @pytest.mark.parametrize('provider_date, expected_output', [
     ('2017-01-21+11%3A56%3A11', datetime(2017, 1, 21, 11, 56, 11)),
     ('2017-05-21+11%3A56%3A11', datetime(2017, 5, 21, 10, 56, 11))