Add migration to compute sponsorship hash in existent coop candidates
requested to merge feat/migration-create-sponsorship-hash-existent-coop_candidate into feat/sponsor-process-first-iteration
CHANGELOG:
### Added
- [#804](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/804) Add migration to compute sponsorship hash in existent coop candidates
Related card: https://trello.com/c/3w06YJRW/1256-candidat-a-s%C3%B2cia-no-esta-generant-codic-per-apadrinament
Edited by Pelayo García