[IMP] l10n_ch: deconfuse BVR implementation

The BVR implementation was confusing as it
seemed to only take into account the case where the postal
account belonged to PostFinance itself and not to another
bank, where the same number is shared among multiple accounts.
Also, the purpose of the fields was confusing.

Adaptations done:
- When you add a bank account of type postal in the account number,
there is an onchange that automatically puts it in the postal account
field and makes itself unique by adding the name of the partner to
the account number. The account will still be detected as "postal".
- Improve help message of the postal account field on the bank account
as it is used for two different purposes (Client Number for own accounts
and postal number for "postal" vendor accounts)
- Only show the subscription fields when you have to do with a bank
account related to your own accounts / Switzerland
- Add fields in the Add Bank Account wizard if it is for Switzerland
(subscription number and the postal field we label "Client Number")
- Also label Client Number on the journal form for postal field
- Don't remove zeros in front of the client number on the BVR
- Don't put extra - in the subscription number on the BVR if it has
some - already
- Use number without spaces for payment reference
- For the CAMT import, it is also possible that we need to split
the account number, because of their non-uniqueness.

Task 2082050