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)

app/notifications/process_notifications.py

@@ -12,6 +12,7 @@ from notifications_utils.recipients import (
 
 from app import redis_store
 from app.celery import provider_tasks
+from app.celery.letters_pdf_tasks import create_letters_pdf
 from app.config import QueueNames
 
 from app.models import (
@@ -149,6 +150,10 @@ def send_notification_to_queue(notification, research_mode, queue=None):
         if not queue:
             queue = QueueNames.SEND_EMAIL
         deliver_task = provider_tasks.deliver_email
+    if notification.notification_type == LETTER_TYPE:
+        if not queue:
+            queue = QueueNames.CREATE_LETTERS_PDF
+        deliver_task = create_letters_pdf
 
     try:
         deliver_task.apply_async([str(notification.id)], queue=queue)