waitlist notifications

[jhdaws]

Added waitlist notifications (and some waitlist functionality so it made more sense)

[yifeifang11]

tested both successful addition to waitlist and shifting of waitlist when someone leaves the course

[yifeifang11]

jack added endpoint functionality for waitlist addition and shifting when someone leaves a full course. he added dropCourseEnrollment function in the course controller, and modified the register endpoint in the user controller to allow for waitlist addition. email notifications also added but yet to be tested because our email free trial ended oops.

i added a button for addition to waitlist in the course catalog and a leave course button in the course a user is already registered for. i modified the user type in the frontend to allow for the waitlist to be imported in the catalog page so that the frontend can detect when the user is added to a waitlist to give user feedback and also to prevent double waitlist registration. the leave course button added to the course page that calls jack's endpoint successfully.

we will test email notifications later!