Refactor out function to get page count

This has been moved to the letters utils file since it will be used in
more than one place. The notification parameter has been removed so that
the function can be used when we don't have a notification id.

app/letters/utils.py

@@ -1,3 +1,5 @@
+import io
+import math
 from datetime import datetime, timedelta
 from enum import Enum
 
@@ -5,6 +7,7 @@ import boto3
 from flask import current_app
 
 from notifications_utils.letter_timings import LETTER_PROCESSING_DEADLINE
+from notifications_utils.pdf import pdf_page_count
 from notifications_utils.s3 import s3upload
 from notifications_utils.timezones import convert_utc_to_bst
 
@@ -201,3 +204,10 @@ def letter_print_day(created_at):
     else:
         print_date = bst_print_datetime.strftime('%d %B').lstrip('0')
         return 'on {}'.format(print_date)
+
+
+def get_page_count(pdf):
+    pages = pdf_page_count(io.BytesIO(pdf))
+    pages_per_sheet = 2
+    billable_units = math.ceil(pages / pages_per_sheet)
+    return billable_units