Extract template / csv utility code into modules

This follows a similar approach to the previous commits, noting that
one module depends on the other, so we have to extract both together.

tests/app/main/test_errors_for_csv.py

@@ -2,7 +2,7 @@ from collections import namedtuple
 
 import pytest
 
-from app.utils import get_errors_for_csv
+from app.utils.csv import get_errors_for_csv
 
 MockRecipients = namedtuple(
     'RecipientCSV',