Ambassadors have analytic accounts and portal users.

- [FIX] Origin and Channel are not editable in active states
- [NEW] All portal users have an associated analytic account
Make date fields of contract more clear.

- Change names of dates
- Compute automatically next_invoice_date of new contracts
Merge branch 'sync-contracts-gp' of https://github.com/CompassionCH/compassion-modules into sync-contracts-gp