Create a solid model.conf to test Casbin with a use case close to what we'll implement

[MaferMazu]

In #31 we have the foundations, and #37 we had our first test of the technology. The idea is based on the feedback from those PR and also the https://openedx.atlassian.net/wiki/spaces/OEPM/pages/5216927745/How+to+Model+model.conf+and+authz.policy doc, having a solid model.conf to start working on.

[mariajgrimaldi]

We already started working on a baseline model in this POC #37 and this document about how this should look like, so we decided to move forward and implement an initial version for the model that complies with what we need for the MVP. @BryanttV will eventually publish our approach in this task. Thanks!