Move ISR subscription fields under res.partner.bank

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>