w 7pm b replace existing classes & schedule remove button

[Zonkman]

the "add" method of CourseController has been changed to replace the old lecture and section, if it finds an old one.

also, because of moving common delete functionality to another function, and it having already existed in the delete method, this absorbed https://github.com/ucsb-cs56-w20/ucsb-courses-search/pull/237 . so this request also made the delete button on the schedule page for each class, that when pressed, removes the lecture and section together from the schedule.

[Zonkman]

merge conflicts fixed, my methods now use the new names, and functionality combined

[agprice]

When trying to add a class, a whitelabel error page is produced.