As an admin, I can post media reviews by pasting the link

Admins can now post reviews by pasting a link instead of copying the review title, author, and article text. Given the URL, we use the Alchemy API to extract the title and author, and then run sentiment analysis on the review text, also via the URL.

After creating a review, we display it so the user can verify that the API correctly parsed the title, author, and text.

Also, since Alchemy's sentiment analysis returns a single value, we've removed the two unused sentiment columns from `media_reviews`.

**Setup Alchemy API**
* Add Alchemy Gem to `Gemfile`
* Add Alchemy API key to `.env.sample`
* Create initializer for Alchemy config

**Refactor application to use Alchemy for sentiment analysis**
* Update `SentimentAnalyzer` to use Alchemy API
* Remove unnecessary columns from `media_reviews`
* Refactor `MediaReview` for updated `SentimentAnalyzer` and single sentiment column
* Remove unused sentiment fields from `media_review` partial
* Create `admin/media_reviews#create` view to display the saved review
* Remove unused review text field from `admin/media_reviews#new`

**Extract author and title metadata from a given URL**
* Create `UrlMetadataExtractor` class to extract author and title via URL
* Refactor `MediaReview` and `Admin::MediaReveiwsController` to use metadata extractor
* Remove `title` and `author` fields from `admin/media_reviews#create` form
* Retain these fields on `admin/media_reviews#edit` form to allow users to make corrections

https://trello.com/c/o9o7dA63
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