Activity now assigned to current user

CHANGELOG:

### FIXED
- [#610](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/610) Activity now assigned to current user

Related card: https://trello.com/c/p2o3v9qn/841-assignaci%C3%B3-dactivitats-de-retorn-a-factura

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