Make change tariff button visible in broadband contract view

CHANGELOG:

### Changed
- [#741](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/741) Make change tariff button visible in broadband contract view

Related card: https://trello.com/c/meyu2G7m/1167-el-bot%C3%B3-canvi-de-tarifa-shauria-de-veure-en-contractes-ba

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