Format auth_type in a consistent way in the UI

On the ‘find user’ page it says ‘sms_auth’ instead of ‘Text message
code’.

This commit fixes that, and adds a handy formatter so it’s easier to do
the right thing in the future.

tests/app/main/views/test_find_users.py

@@ -120,7 +120,7 @@ def test_user_information_page_shows_information_about_user(
     ] == [
         'test@gov.uk',
         '+447700900986',
-        'sms_auth',
+        'Text message code',
         'Last logged in just now',
     ]