give test letter api notifications a different filename

so they can be distinguished on the frontend.

Also, some related cleanup:

* don't show test api letters on the frontpage
* make sure the subject is returned from the API for letters
* make sure the letter's address is returned for letters

app/dao/notifications_dao.py

@@ -338,7 +338,8 @@ def get_notifications_for_service(
         filters.append(Notification.created_at < older_than_created_at)
 
     if not include_jobs or (key_type and key_type != KEY_TYPE_NORMAL):
-        filters.append(Notification.job_id.is_(None))
+        # we can't say "job_id == None" here, because letters sent via the API still have a job_id :(
+        filters.append(Notification.api_key_id != None)  # noqa
 
     if key_type is not None:
         filters.append(Notification.key_type == key_type)