[RFR] Module description
[RFR] Clarify constraint messages
[RFR] Convention _obj (or _pool) for pools
[FIX] Remove void required=False attribute on fields
[FIX] Don't mask existing _prepare_invoice method defined in the stock module
[FIX] Don't hide invoice type and number for regular invoices
[FIX] Typo in previous commit
[IMP] Apply really high (=low) priority when adding an extra header tag to the invoice form
[FIX] No validation button on customs invoices
[ADD] Verify that a customs invoice state is always draft
[IMP] Apply customs terminology in method name
[IMP] Show separate button for showing an already created customs invoice
[FIX] Change label of the second field as well
[RFR] Remove unused code
[RFR] Reuse more code from the sale module e.g. to make the taxes show up,
and benefit from (actual) inherits of this code.
[RFR] Add the counterpart of the customs_invoice_id on the invoice model
to prevent race conditions in applying the constraints during invoice creation
[ADD] Translations
[FIX] Remove debug statement from previous commit
[FIX] Only create an invoice for the lines and quantities on the picking
Merge pull request #2 from StefanRijnhart/7.0-invoice_customs_improvements

7.0 invoice customs improvements