Make change tariff button available only for IT group

CHANGELOG:

### Changed
- [#604](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/604) Make change tariff button available only for IT group

Related card: https://trello.com/c/CXESfQsS/912-fer-que-el-wizard-de-canvi-de-tarifa-nom%C3%A9s-sigui-visible-i-funcional-pel-grup-it

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