[FIX] payment, website_sale: SIPS adaptation

SIPS payment has two limitations:
- A payment reference can only be used once, even if the payment was not
  proceeded
- A payment reference can only contain alphanumerical characters

These limitations do not fit the payment process which:
- Reuses existing transactions
- Introduces a `-` character when creating a new transaction for the
  same order.

First issue is solved by an ugly hack since there is no link module
between `website_sale` and `payment_sips` in order to override the
behavior of `sale_get_transaction`.

Second issue is solved by modifying the character to `x`. In this case,
we could consider overriding the method `get_next_reference` in
`payment_sips`. However, this method is called before the creation of
the transaction, and we should do a crappy workaround using the context
to get the `acquier_id`.

opw-728209