feat(MailActivity): add partner tags related field

CHANGELOG:

### Added
- [#1015](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1015) Add partner tags related field

Related card: https://trello.com/c/sAdFvUrf/1581-camp-etiqueta-a-la-vista-form-del-model-mailactivity

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