[feat] Create add student endpoint

[yuxiaolejs]

In this Issue you will create a backend endpoint /api/students/post for creating a single student in a certain course.

This is required by PR #46

Front end is not required for now but swagger is in urgent need

Acceptance Criteria

• [x] The end point /api/students/post exists and does its work
• [x] It is shown on swagger page
• [x] It has test coverage

Discussion

This will eventually be used on the Course Show Page #55