Ignore one-off messages in job list on dashboard

Same as how we ignore ‘send yourself a test’ messages (see:
d8467bfc3c). The dashboard gets clogged
up with one off messages otherwise, which affects:
- performance
- users ability to find their jobs

app/config.py

@@ -107,6 +107,7 @@ class Config(object):
     SMS_CHAR_COUNT_LIMIT = 495
     BRANDING_PATH = '/images/email-template/crests/'
     TEST_MESSAGE_FILENAME = 'Test message'
+    ONE_OFF_MESSAGE_FILENAME = 'One-off message'
     MAX_VERIFY_CODE_COUNT = 10
 
     NOTIFY_SERVICE_ID = 'd6aa2c68-a2d9-4437-ab19-3ae8eb202553'