Allow to auto validate holidays

CHANGELOG:

### Added
- [#660](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/660) Allow users to auto-validate their holidays 

Related card: https://trello.com/c/hnBgXo6v/1029-auto-aprovaci%C3%B3-dabs%C3%A8ncies?filter=member:gerardfunosas

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