[12.0][ADD] user_group_readonly

[hbrunn]

This module adds a user type 'Readonly access' that grants readonly access to all parts of Odoo.

It is meant to be used to create accounts for auditors.

To use this module, you need to:

1. Go to Settings/Users & Companies/Users
2. Create a user and choose type 'Readonly access'
3. In case you install/uninstall modules, run cronjob 'Update Readonly Access Group'