CSV Export should have Payee field, not 'payee_name'

[nielsmaerten]

Describe the bug
When exporting transactions to CSV, the CSV should have the following columns:
Payee, Date, Amount, Memo
Source: https://docs.youneedabudget.com/article/921-formatting-csv-file

Currently, the file has these columns:
payee_name, date, amount, memo

Expected behavior
Use fields Payee, Date, Amount, Memo instead

Additional context
Reported by @iofjuupasli: #35