Make fields readonly on subscription request view

CHANGELOG:

### Changed
- [#642](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/642) Make fields readonly on subscription request view

Related card: https://trello.com/c/quqzCVii/985-vista-sr-fer-camps-n%C3%BAmero-daccions-tipus-daccions-i-quota-de-subscripci%C3%B3-no-editables

---