Perform sentiment analysis when creating Media Reviews

* Add fields to `media_reviews`: `headline`, `author`, `source`, and 3 fields for sentiment
* Add presence validations for `headline` and `source` to model
* Add `source`, `headline`, `author`, and `review_text` fields to `media_reviews#new`
* Update `Admin::MediaReviewsController` with new fields
* Update `MediaReview` model with methods for sentiment analysis
* Create `SentimentAnalyzer` class with analysis methods via API
* Display (unfiltered) sentiment values in `_media_review` partial
`MediaReview.analyze_new`

I believe this is as per the [Style Guide](https://github.com/thisismetis/ruby-style-guide#syntax)
> Use :: only to reference constants(this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Never use :: for regular method invocation.