Completing tests for basic course and user apps

Following the code refactor, I wrote tests for the three apps so far - user_auth, courses and registration. The tests cover almost all the non-trivial logic written so far, and so now the time has come to move on to the next part - course lectures.

A course can be created by an admin user and other admins can be added as instructors. Maybe, an option can be created for a teaching assistant at a later stage, in case an instructor does not want to add other co-instructors as admins but rather just other users as helpers. This I will figure out later along with the logic for deleting courses.

A course will have many lectures, so this is a simple case of a foreign key. The lectures can be video or audio or just text. Rather than having the content directly in the lectures, it might be better to create another table for that so that the content can be given metadata such as text translations for other languages or just merely audio in different languages.

In terms of a lecture, it should have a title, a number in a sequence, an optional description, content and optional attachments. Basic operations should be CRUD, except bulk delete should not be allowed in case a malicious instructor tries to delete a course. It should also be possible to change the sequence of lectures in a course. Of course, only an instructor should be able to perform any of these actions.

In terms of student registration, currently any student can register for any course paid or free. The payment processing will come later. In terms of student registrations, my plan is to allow any student to watch around 40% of a course for free and only then ask for payment for those courses which are paid. The reasoning - after watching 40% of the course, a student can judge whether the course is truly relevant and useful, and this way there will be no possibility of refunds later. The way to do this would be to update the CourseStudentRegistration model to include a paid field which can initially be False and will turn to True only when payment is processed. This can be used to check for authorization on whether a student can watch a particular video - if the lecture number is greater than the free quota, he will be asked to pay.