Create endpoint to refresh user schedule

[tomaspalma]

For performance reasons we have a table inside of our database called user_course_units which stores a mapping between the course id and the class id for the class the user is enrolled in.

However, we currently have no mechanism for refreshing sigarra changes that were made outside of our platform.

After we create this endpoint, we can call it from the frontend when the user enters the exchange page. This was chosen to be done like this because we should have things inside of the response handling of our backend in order not to decrease response time.