Perform basic validation of bank account and bank id form data

The form fields directdebit_bank_account_number and directdebit_bank_id should contain only numbers. I have performed a quick research and to my knowledge there are no bank ids that do contain anything except digits or dashes anywhere in the world (a more thorogh reasearch might be needed, though). Unfortunately, the user input for these fields is not validated by directdebit. Please find attached a patch, that implements some basic checks for these fields.