When creating tapir users through CreateGeneralTapirAccountView, invalid characters are not filtered

[Theophile-Madet]

If we try to create a user with an invalid character in the username (we had the case with "krümelbande"), the tapir_user is created but an errors comes from LDAP where the user is then not created. There is then a data inconsistency between the Tapir db and the LDAP db.

• The creation page should refuse invalid characters
• If an error happens when creating the user on the LDAP side, the user on the Tapir side should be deleted from the DB (probably with @transation.atomic)

[Theophile-Madet]

Looks like there's also an issue with family names being required by LDAP but not by the form.