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_sms.csv

@@ -1,4 +1,4 @@
-phone number,name
+PhoneNumber,Name
 +441234123121,chris
 +441234123122,chris
 +441234123123,chris