Additions to the lecture API endpoints

With the basic CRUD for lecture endpoints done, I will need to plan some other details as well as the lecture contents and resources (or attachments). For now, a lecture must have a title which must be unique in a course, and an optional description. Another field that a lecture must have is whether a preview is enabled, and in that case anyone can view it for free. Also, if the lecture has video content, the duration of the video needs to be displayed even in the list view.

Next comes the sequence of lectures in a course. There has to be a sequence ID to indicate how the lectures are to appear, and an instructor can move lectures around and change the sequence. This sequence ID is also unique within a course, and must not be set directly by the instructor, but rather must be generated during the creation process. In the case of a single lecture creation, it might be fairly easy, as one only needs to check the largest sequence number in the course and add the new lecture. In the case of bulk lecture creation, it might need some kind of counter as the contents of a spreadsheet might be read. Finally, in the lecture list view, the lectures should appear in the ascending order of the sequence ID.

There has been no difference between the Lecture List view and the Detail view. However, these two should be different - the list view should only display the title, description and time duration of a lecture. The detail view should provide everything - details of the video content, resources for download etc.

Some changes such as the detail view can wait until the other models such as videos and downloads are ready. But the sequence ID can be handled right away. Along with creating the sequence ID and retrieving lectures according to sequence ID, an API call for changing the sequence ID of a lecture must also be created. So, if lecture 15 in a course needs to be inserted before lecture 9, the sequence ID of lecture 15 will become 9, and the sequence IDs of lectures 9 to 14 will increase by 1. And the reverse process if a lecture needs to be moved down the list.