Mark letters as validation-failed if the templated letter is too long.

It is possible that the personalisation for a templated letter can make the letter exceed 10 pages or 5 sheets. We are not validating the letters posted via the API for this validation error. It is only possible to validate the letter once we create the PDF in notifications-template-preview. This means that the letter can only get a validation-failed status after the client has received a 201 from the POST to /v2/notifications.
NOTE: we only validate the preview row of a CSV for this validation error, this change will mean that it is possible for a letter to be marked as validation-failed after a successful file upload.

A new task to update the notification to `validation-failed` has been added to the API. If we find that the letter is too long once we have created the PDF we call the `update-validation-failed-for-templated-letter` task rather than `update-billable-units-for-letter` task.

New work flow for a letter in brief:
API - receives POST /v2/notifications
:: save to db
:: put CREATE_LETTERS_PDF task on queue for template preview to consume
TEMPLATE-PREVIEW - consumes task CREATE_LETTERS_PDF
:: create PDF
:: count pages of PDF
:: IF page count exceeds 10 pages
	 put in the letters-invalid-pdf S3 bucket with metadata (similar to the precompiled letters)
	 put `update-validation-failed-for-templated-letter` task on the queue for the API to consume
   ELSE
     put PDF in the `letters-pdf` bucket
     put `update-billable-units-for-letter` task on the queue
API - consumes `update-billable-units-for-letter` OR `update-validation-failed-for-templated-letter` task
:: IF `update-billable-units-for-letter` task:
   	update billable units for notification as usual
:: ELSE `update-validation-failed-for-templated-letter`:
   	update notification_status = `validation-failed`
ADMIN - view notification page for letter
:: show validation letter for templated letter

There will be 3 PRs in order to make this change, one for the API, template-preview and the admin app.

Deployment plan

Deploy Admin first
Deploy API
Deploy template-preview

Related PRs:
alphagov/notifications-template-preview#619
alphagov/notifications-admin#4107

https://www.pivotaltracker.com/story/show/169209742

app/v2/notifications/post_notifications.py

@@ -19,7 +19,6 @@ from app.celery.letters_pdf_tasks import (
     get_pdf_for_templated_letter,
     sanitise_letter,
 )
-from app.celery.research_mode_tasks import create_fake_letter_response_file
 from app.celery.tasks import save_api_email, save_api_sms
 from app.clients.document_download import DocumentDownloadError
 from app.config import QueueNames, TaskNames
@@ -389,11 +388,6 @@ def process_letter_notification(
         queue=queue
     )
 
-    if test_key and current_app.config['NOTIFY_ENVIRONMENT'] in ['preview', 'development']:
-        create_fake_letter_response_file.apply_async(
-            (notification.reference,),
-            queue=queue
-        )
     resp = create_response_for_post_notification(
         notification_id=notification.id,
         client_reference=notification.client_reference,