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

tests/app/v2/notifications/test_post_letter_notifications.py

@@ -296,7 +296,7 @@ def test_post_letter_notification_with_test_key_creates_pdf_and_sets_status_to_d
     'development',
     'preview',
 ])
-def test_post_letter_notification_with_test_key_creates_pdf_and_sets_status_to_sending_and_sends_fake_response_file(
+def test_post_letter_notification_with_test_key_creates_pdf(
         notify_api, client, sample_letter_template, mocker, env):
 
     data = {
@@ -312,8 +312,6 @@ def test_post_letter_notification_with_test_key_creates_pdf_and_sets_status_to_s
     }
 
     fake_create_letter_task = mocker.patch('app.celery.letters_pdf_tasks.get_pdf_for_templated_letter.apply_async')
-    fake_create_dvla_response_task = mocker.patch(
-        'app.celery.research_mode_tasks.create_fake_letter_response_file.apply_async')
 
     with set_config_values(notify_api, {
         'NOTIFY_ENVIRONMENT': env
@@ -323,7 +321,6 @@ def test_post_letter_notification_with_test_key_creates_pdf_and_sets_status_to_s
     notification = Notification.query.one()
 
     fake_create_letter_task.assert_called_once_with([str(notification.id)], queue='research-mode-tasks')
-    assert fake_create_dvla_response_task.called
     assert notification.status == NOTIFICATION_SENDING