#162163273 Users should be able to favorite articles.

[joshtrigger]

What does this PR do?
This Pull Request enables a user to favorite an article

Description of tasks
Below are the endpoints that enable the user have the access to favorite or unfavorite an article
POST /api/articles/<slug>/favorite/
DELETE /api/articles/<slug>/favorite/

How to manually test it

• clone the repository

• install the dependencies in the requirements.txt file

• the POST /api/articles/<slug>/favorite/ should return

{
  "article": {
    "slug": "how-to-train-your-dragon",
    "title": "How to train your dragon",
    "description": "Ever wonder how?",
    "body": "It takes a Jacobian",
    "tagList": ["dragons", "training"],
    "createdAt": "2016-02-18T03:22:56.637Z",
    "updatedAt": "2016-02-18T03:48:35.824Z",
    "favorited": true,
    "favoritesCount": 2,
    "author": {
      "username": "jake",
      "bio": "I work at statefarm",
      "image": "https://i.stack.imgur.com/xHWG8.jpg",
      "following": false
    }
  }
}

• the DELETE /api/articles/<slug>/favorite/ should return

{
 "message":{
  "body":[
    "article has been unfavorited"
  ]
 }
}

Relevant pivotal tracker stories
#162163273

[Kasulejoseph]

@joshtrigger you have some conflicts to fix