Write tests for code generation

[lbkchen]

One of the main things to test would be to ensure that the courses that you can generate codes for (in GenerateCodeCsvModal) are all published, so you can't sign a student up for a draft course. Also ensure that you can't generate anything dangerously (ex. no courses selected, no name given, too many/few codes, etc.). Can't think of anything else atm but let me know if you do!