stream notifications when collating zip files

we had issues where we had 150k 2nd class notifications, and the collate
task never ran properly, presumably because the volume of data being
returned was too big.

to try and help with this, we can switch to streaming rather than using
`.all` and building up lists of data. This should help, though the
initial query may be a problem still.

app/celery/letters_pdf_tasks.py

@@ -178,7 +178,6 @@ def collate_letter_pdfs_to_be_sent():
 
 def get_key_and_size_of_letters_to_be_sent_to_print(print_run_deadline, postage):
     letters_awaiting_sending = dao_get_letters_to_be_printed(print_run_deadline, postage)
-    letter_pdfs = []
     for letter in letters_awaiting_sending:
         try:
             letter_file_name = get_letter_pdf_filename(
@@ -188,17 +187,15 @@ def get_key_and_size_of_letters_to_be_sent_to_print(print_run_deadline, postage)
                 postage=letter.postage
             )
             letter_head = s3.head_s3_object(current_app.config['LETTERS_PDF_BUCKET_NAME'], letter_file_name)
-            letter_pdfs.append({
+            yield {
                 "Key": letter_file_name,
                 "Size": letter_head['ContentLength'],
                 "ServiceId": str(letter.service.id)
-            })
+            }
         except BotoClientError as e:
             current_app.logger.exception(
                 f"Error getting letter from bucket for notification: {letter.id} with reference: {letter.reference}", e)
 
-    return letter_pdfs
-
 
 def group_letters(letter_pdfs):
     """