Replace test with one-off

It’s a confusing proposition to have two features which are almost
identical. Even differentiating between them in the template menu would
be tricky.

I think the better thing to do is rename the whole feature to ‘send
one-off message’.

Then if someone wants to use there own phone number or email address,
give them a quick shortcut to doing that, once they’re in the flow.
In the background this reuses the ‘send yourself a test’ code, but
the user is never aware that they’re going through a different route
to send an individual message. So the proposition stays nice and clean.

app/config.py

@@ -57,7 +57,7 @@ class Config(object):
     CSV_UPLOAD_BUCKET_NAME = 'local-notifications-csv-upload'
     DESKPRO_PERSON_EMAIL = 'donotreply@notifications.service.gov.uk'
     ACTIVITY_STATS_LIMIT_DAYS = 7
-    TEST_MESSAGE_FILENAME = 'Test message'
+    TEST_MESSAGE_FILENAME = 'One-off message'
 
     STATSD_ENABLED = False
     STATSD_HOST = "statsd.hostedgraphite.com"