Master bug fixes

Small functionality issues corrected

- [FIX] BVR Reference generation including creating a different reference for a new group.
- [FIX] Workflow of contract (clean invoices on cancellation), plus addition of
           methods for having the correct workflow state after the export from GP.
- Payment terms restricted to those used by Compassion, set "Permanent Order" as default term.
- Various bug fixes

Note : ignore the commit message regarding the default date of contract. Currently, the start_date is the current day by default. It is planned for the future to compute the start_date and more importantly the next_invoice_date on creation of a contract.