Upload pre-compiled letter PDF to S3

Pre-compiled letter endpoint uploads PDF contents to S3 directly
instead of creating a letter task to generate PDF using template
preview.

This moves some of the utility functions used by existing letter
celery tasks to app.letters.utils, so that they can be reused by
the API endpoint.

app/letters/utils.py

@@ -0,0 +1,50 @@
+from datetime import datetime, timedelta
+
+from flask import current_app
+
+from notifications_utils.s3 import s3upload
+
+from app.variables import Retention
+
+
+LETTERS_PDF_FILE_LOCATION_STRUCTURE = \
+    '{folder}/NOTIFY.{reference}.{duplex}.{letter_class}.{colour}.{crown}.{date}.pdf'
+
+
+def get_letter_pdf_filename(reference, crown):
+    now = datetime.utcnow()
+
+    print_datetime = now
+    if now.time() > current_app.config.get('LETTER_PROCESSING_DEADLINE'):
+        print_datetime = now + timedelta(days=1)
+
+    upload_file_name = LETTERS_PDF_FILE_LOCATION_STRUCTURE.format(
+        folder=print_datetime.date(),
+        reference=reference,
+        duplex="D",
+        letter_class="2",
+        colour="C",
+        crown="C" if crown else "N",
+        date=now.strftime('%Y%m%d%H%M%S')
+    ).upper()
+
+    return upload_file_name
+
+
+def upload_letter_pdf(notification, pdf_data):
+    current_app.logger.info("PDF Letter {} reference {} created at {}, {} bytes".format(
+        notification.id, notification.reference, notification.created_at, len(pdf_data)))
+
+    upload_file_name = get_letter_pdf_filename(
+        notification.reference, notification.service.crown)
+
+    s3upload(
+        filedata=pdf_data,
+        region=current_app.config['AWS_REGION'],
+        bucket_name=current_app.config['LETTERS_PDF_BUCKET_NAME'],
+        file_location=upload_file_name,
+        tags={Retention.KEY: Retention.ONE_WEEK}
+    )
+
+    current_app.logger.info("Uploaded letters PDF {} to {}".format(
+        upload_file_name, current_app.config['LETTERS_PDF_BUCKET_NAME']))