[FIX] account: fix mass mail wizard

- In the sales or accounting module, create some invoices.
- Select them then click on `action > send & print`
- The wizard opens but not in the mass mail mode.

It creates some inconsistency and if you click on send,
the invoices are sent to the wrong customer.

Sometimes, the invoices are not sent at all.

I think the problem comes from the `_compute_composition_mode` method
who set the value of `composition_mode` on the wizard instead of
on the composer.

OPW-2085837