7.0 debit usability

The first commit fixes the following issues with SEPA mandates:
- the mandate field is not only visible on the payment lines on debit orders
- the mandate is not copied from the invoice because of recent client changes (purging default_* keys in form view)
- the mandate is not copied from the invoice when creating a debit order from the payment order menu item instead of the debit order menu order. Make it impossible to do so.
- payment lines can be edited from the mandate view.

The second commit is just a PEP8 refactoring of the affected modules

The third commit sets the admin language to English during testing.