Add child state in tree view...

- [NEW] Child state in tree view, with colors
- [NEW] Child filters and search options
- [FIX] Bug on end wizard, transfer_country field required only if checkbox
  is selected.
- [FIX] Only available children can be selected for new sponsorships
- [NEW] Remove quick create options from contract view
Track changes on children

- [NEW] Added mail thread in view of children
- [NEW] Updated the tracked fields
- [FIX] Sponsor_id field is not editable
New child exit details based on GP ending reasons.

- [NEW] Child exit_details as they are in GP.
- [FIX] Don't set a sponsor when contract is edited and child removed.
- [FIX] Children in 'waiting new sponsor' state can be sponsored :)
- [FIX] Don't update twice the state of the child.