move generate_pdf_letter func to utils app

src/shop/invoiceworker.py

@@ -1,5 +1,5 @@
 from django.core.files import File
-from shop.pdf import generate_pdf_letter
+from utils.pdf import generate_pdf_letter
 from shop.email import add_invoice_email, add_creditnote_email
 from shop.models import Order, CustomOrder, Invoice, CreditNote
 import logging

src/tickets/models.py

@@ -13,7 +13,7 @@ from utils.models import (
     UUIDModel,
     CreatedUpdatedModel
 )
-from shop.pdf import generate_pdf_letter
+from utils.pdf import generate_pdf_letter
 
 logger = logging.getLogger("bornhack.%s" % __name__)