Process CSV files with column headers of any case

Implements and depends on:
https://github.com/alphagov/notifications-utils/pull/27

Should be merged and deployed before the same change in the admin app.

test_csv_files/multiple_email.csv

@@ -1,4 +1,4 @@
-email address
+EMAILADDRESS
 test1@test.com
 test2@test.com
 test3@test.com