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.

tests/app/main/views/test_dashboard.py

@@ -172,6 +172,25 @@ def test_inbox_showing_inbound_messages(
     )
 
 
+def test_inbox_handles_escaped_characters(
+    client_request,
+    service_one,
+    mock_get_inbound_sms_with_special_characters,
+):
+
+    service_one['permissions'] = ['inbound_sms']
+
+    page = client_request.get('main.inbox', service_id=SERVICE_ONE_ID)
+
+    assert normalize_spaces(
+        str(page.select_one('tbody tr .file-list-hint'))
+    ) == (
+        "<span class=\"file-list-hint\">"
+        "the first line's content the second line's content"
+        "</span>"
+    )
+
+
 def test_empty_inbox(
     logged_in_client,
     service_one,