fix: remove legacy authz policy and action grouping

[mariajgrimaldi]

Description

Static role-permission policies now come from the authorization schema, so this removes authz.policy along with g2 action grouping from the Casbin model and deletes the existing g2 rows through a data migration. The built-in roles already list every permission covered by those relationships and the tests now verify those explicit assignments, which means this is not a breaking change for supported configurations.

Permission grouping and role inheritance remain outside the authorization schema, with that work tracked in #91 and #523.

load_policies remains available because Tutor still calls it during LMS and CMS initialization, although calls without a policy file now do nothing. Open PR overhangio/tutor#1496 adds load_authz_schema, so it can be extended to remove the load_policies calls or overhangio/tutor#1509 can make that change separately. Issue #539 tracks whether the explicit custom-file workflow remains supported as is and whether the command should be removed after the Tutor change is released.

Solves #462

How to Test

Run:

python manage.py migrate
python manage.py load_policies
python manage.py load_authz_schema --dry-run --dir openedx_authz/authz/schema
python manage.py load_authz_schema --dir openedx_authz/authz/schema

These commands run as usual without loading policy rows from a file. In a clean database, load_policies exits successfully without a policy file, the schema command loads 132 p rows and no g2 rows, and a second run reports no changes.

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[rodmgwgu]

Thanks for this work, I have some concerns about removing g2 support tough:

This is what I had in mind originally for this ticket: Just remove the p entries from authz.policy and keep g2 for now as this is not represented in yaml currently.

Then, on #523 I was planning on modeling the permission dependencies in the yaml schema and update the compiler so it would output g2 rows based on the yaml definition.

The idea was that we would use the information from those rows to imply dependencies to be latter shown to the end user in the UI wizard when defining custom roles, so it's clear that enabling one permission implies others (when defined).

Alternatively if we remove g2, then defining the permission dependencies in YAML wouldn't have any effect at the casbin level, but only on the UI level which would then assign each permission and its dependencies one by one to a role.

I'm not 100% sure what's the best path forward, what do you think?