Allow voting on reviews

First draft, posting a PR mainly since I have a few questions, and for some early feedback. 
![rate_reviews_1](https://cloud.githubusercontent.com/assets/2644780/3867707/5265ae9c-200b-11e4-83eb-ac71a6d84bc3.png)

nit: We already use the verb "rated" to refer to a user reviewing and assigning scores to a course/prof along interest, clarity, etc.. It might be clearer to use a different verb like "voted" to refer to whether a user upvoted or downvoted a review.

Also, I think it might be cleaner to maintain two separate list: `voted_course_review_ids` and `voted_prof_review_ids` rather than mixing both types of reviews into one list.