The IBAN should not be required upon registration

[SergioArbarviro]

The IBAN is currently required upon registration:

https://github.com/michaellaunay/alirpunkto/blob/05f3c56d7bfd1462635bc0cfbaa19c9fbf2c317b/alirpunkto/schemas/register_form.py#L202

The opposite is true. This information is useful only if and when the Cooperator anticipates to leave the Cooperative, for the Cooperative to transfer to that bank account the sums resulting from the back purchase of the Cooperator's shares. For security reasons, it should hence be provided as late as possible by the Cooperator.

I propose that this variable be made not required, and that the corresponding msgid "iban_required" be suppressed.

[SergioArbarviro]

Duplicates partially #94