Allow users to upvote/downvote reviews

[nathanmsmith]

Right now, we display all reviews for courses in a random order. It'd be nice if we could sort reviews by "helpfulness", where helpfulness is defined as the most upvotes a review has.

A user must be logged in in order to up/downvote. We should have a modal pop-up telling users to log in if they aren't logged in.

There will be a frontend and backend component to this feature.

Frontend

Add upvote and downvote icons to icons. These icons should trigger AJAX requests so that the review can be voted upon. It may make sense to convert the review component to React. Talk to me and @TimothyGu about this.

Order the reviews to be sorted by upvotes.

Backend

Update the reviews modal to allow it to have up and down votes. This could be done via a gem or a home-rolled solution. Talk to me and @TimothyGu about the tradeoffs of each before implementing. See the following:

• https://gorails.com/episodes/liking-posts
• https://github.com/ryanto/acts_as_votable