Allow the admin app to send one-off letters

This commit modifies the code paths the admin app uses to send one off
emails and text messages to also accept letters.

This mostly worked already, the two changes were:
- making sure that one-off letters are processed by the correct task,
  from the correct queue
- one-off letters sent from a service in research mode don’t get put on
  a queue and go straight to `delivered` (because we don’t want to send
  them for real)

tests/app/service/test_send_one_off_notification.py

@@ -258,6 +258,22 @@ def test_send_one_off_letter_notification_should_use_template_reply_to_text(samp
     assert notification.reply_to_text == "Edinburgh, ED1 1AA"
 
 
+def test_send_one_off_letter_should_not_make_pdf_in_research_mode(sample_letter_template):
+
+    sample_letter_template.service.research_mode = True
+
+    data = {
+        'to': 'A. Name',
+        'template_id': str(sample_letter_template.id),
+        'created_by': str(sample_letter_template.service.created_by_id)
+    }
+
+    notification = send_one_off_notification(service_id=sample_letter_template.service.id, post_data=data)
+    notification = Notification.query.get(notification['id'])
+
+    assert notification.status == "delivered"
+
+
 def test_send_one_off_sms_notification_should_use_sms_sender_reply_to_text(sample_service, celery_mock):
     template = create_template(service=sample_service, template_type=SMS_TYPE)
     sms_sender = create_service_sms_sender(