[ADD] backport sepa direct debit

This is a backport of account_banking_pain_base, account_banking_sepa_direct_debit and account_banking_payment_export. Creating mandates and exporting payment order to PAIN works, but up to now I couldn't look into possible side effects of installing account_banking_payment_export
Not exactly, because import in headers are executed at server initialization time, avoiding to start the server if the library is not present. With this trick, you allow to start server, but don't let the module install with the declaration in `external_dependencies`.