Convert the professor field on Review from StringField to ForeignKey(Instructor)

[cheniel]

Currently we have two sources for professors:

• Through the professor StringField on Review, populated by review writers.
• Through Instructor, which has a ManyToMany relationship to CourseOffering and is crawled through the timetable.

We would like to consolidate these two sources by changing professor on Review from StringField to ForeignKey. This will require a data migration.

This will consolidate the queries on the course_detail and course_professors views. ReviewForm will have to be modified as well.