Remove non-valid postcode and town from Loan

### Description

This is a small simplification to the loan entry postcode entry process.

Currently, if there is an issue with the applicant's postcode, we ask for the lender's branch postcode and some extra detail (town and potentially non-valid postcode) at the bottom of the form. This extra detail isn't proving to be useful so this change removes them from the form and other places in the system where they are exposed (reports for example).

### Notes

The diff is a little noisy because I updated two csv export specs to use column headings instead of column numbers when comparing data. I believe this is a more robust approach and it made it easier to make changes to the csv export code.

I've left the database columns in place for now, which leaves us with the option to extract old data if required.