An Inpayment Slip with Reference (ISR) subscription number is provided by the bank or by Postfinance
ISR When the ISR subscription comes from Postfinance it is a direct
subscription.
ISR (bank) When the ISR subscription comes from a bank it is the bank subscription
toward Postfinance.

Having the subscription number on res.bank has 2 drawbacks:

If you have multiple accounts at Postfinance
or
If you have multicompany and each company has a Postfinance account

you won't be able to emit with different ISR subscriber numbers.

This also renames the fields to be accurate on the meaning.
An ISR reference is a Payment reference that will apear on the invoice.
It is better to name it subscription number.

closes odoo/odoo#35873

Signed-off-by: Josse Colpaert <jco@openerp.com>