Separate name and subject in Mass Mailing

CHANGELOG:

### CHANGED
- [#826](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/826) Separate name and subject in Mass Mailing

Related card: https://trello.com/c/eDG3DkPv/1288-mass-mailing-fer-el-camp-nom-origen-editable-desvincultat-de-lassumpte

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