Switzerland - Fix ISR reference

This module corrects the way the ISR (Swiss orange payment slip) reference is built on customer invoices. It changes the invoice fields that hold the ISR subscription number, which identifies your company or your bank for generating ISR slips.

• The subscription number is now taken from the bank account selected on the invoice (partner_bank_id), using the EUR subscription number when the invoice currency is EUR and the CHF one when it is CHF. If the invoice uses another currency, no subscription number is set.
• Two computed fields are provided: the raw subscription number, padded to the expected length, and a formatted version with dashes and without leading zeros, meant for the ISR report.
• The ISR reference itself is recomputed from the invoice number: non-digit characters are removed, the number is padded with zeros, and a check digit is added. When an ISR-B customer ID exists on the bank account, it is placed at the start of the reference.
• If the invoice has no subscription number or no invoice number, the reference is left empty.

Everything happens automatically on the invoice; there are no new menus or screens. It affects account.invoice and relies on the Swiss bank account data from the localisation modules.