Add a generic kind of banking mandate

This pull request splits the account_banking_sepa_direct_debit in 2 modules. The first one is a generic banking mandate module (account_banking_mandate) and the second one is the account_banking_sepa_direct_debit where sdd_mandate now inherits from account_banking_mandate. 

The purpose to this refactoring is to be able to use banking mandates in other use cases such as CH-localized direct debit or any other banking process that requires an authorization.
Isn't partner_id a mandatory field ?