Warning for Occupied Room on Courses page

**[Original report](https://bitbucket.org/laborstudents/cas-flask/issue/256) by Katelyn Adams (Bitbucket: [paesh1997](https://bitbucket.org/paesh1997), GitHub: [paesh1997](https://github.com/paesh1997)).**

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Registrar needs to see if a room is already taken when assigning it in the courses tab.
Menu can be accessed after clicking "Edit Course"

Currently, there is no indication to whether the room is taken or not. Maybe a warning modal should show up if it is?

Edit:
In addition to throwing an error after choosing a room that's been taken, we should indicate in the dropdown menu that the room has already been claimed by another course at that time.