#161966917 Implement article rating functionality

[codeplus254]

What does this PR do?

• Implement article rating functionality.

  Description of Task to be completed?

• Write article rating tests. They should be failing.
• Create an endpoint for rating an individual article
• Implement article rating functionality
• Make the tests pass.

  How should this be manually tested?

  1. git clone -b ft-users-able-rate-article-161966917 https://github.com/andela/ah-infinity-stones and $ cd ah-infinity-stones
  2. Create and activate virtual env
• virtualenv -p python3 venv
• $ source venv/bin/activate
  3. $ pip3 install -r requirements.txt to install the dependencies
  4. Setup environment variables. Copy all from below and paste them into the .env file. Update the values to your choice.

     export DATABASE_USER='postgres'
     export DATABASE_PASSWORD='postgres'
     export HOST='localhost'
     export PORT='5432'
     export DATABASE_TEST='test_authors_haven'
     export EMAIL_HOST_USER='Infinites'
     export EMAIL_HOST_PASSWORD='st1fflers'
     export SECRET_KEY='my secret my choice'

     Activate the env variables by $source .env.

  5. Run migrations: $ python3 manage.py migrate
  6. Run app - $ python3 manage.py runserver
  7. Acess http://127.0.0.1:8000/api/users/ through Postman, provide inputs in the following format and create account.

     { "user": {
     "username": "your name",
     "email": "your@email.com",
     "password": "@Password321"  
     }
     }

     Check your email and activate the account. Use the registration token to access the following endpoints:

• http://127.0.0.1:8000/api/articles - Post an article.

  
  {
  "title": "How to make it rain",
  "description": "Ever wonder how?",
  "body": "You have to pray",
  "tag": ["reactjs", "angularjs", "rain"]

}

-  To rate an article, copy and paste the slug ( in this case `how-to-make-it-rain`) into the url and make a POST request to `http://127.0.0.1:8000/api/articles/<slug>/rate` (eg. ` http://127.0.0.1:8000/api/articles/how-to-make-it-rain/rate` ) with the following data:

{ "rating": "4" }

- To view all the ratings for a particular post use the same url but make a GET request.
      `http://127.0.0.1:8000/api/articles/<slug>/rate` (eg. ` http://127.0.0.1:8000/api/articles/how-to-make-it-rain/rate` ) 
- In order to view the article's rating average simply visit the following url(not forgetting your slug).
`http://127.0.0.1:8000/api/articles/<slug>` (eg. ` http://127.0.0.1:8000/api/articles/how-to-make-it-rain` ) 

8. Run tests`python3 manage.py test authors.apps.articles.tests.test_views.ArticleRatingTestCase`
#### What are the relevant pivotal tracker stories?
[#161966917](https://www.pivotaltracker.com/story/show/161966917)
#### Screenshots
<img width="1125" alt="screenshot 2018-12-19 at 20 58 26" src="https://user-images.githubusercontent.com/16904946/50252083-f217e080-03f6-11e9-8a02-bc08829acf73.png">
<img width="1118" alt="screenshot 2018-12-19 at 21 00 46" src="https://user-images.githubusercontent.com/16904946/50252086-f3e1a400-03f6-11e9-8f02-e3088e2b8abb.png">
<img width="1119" alt="screenshot 2018-12-19 at 21 01 05" src="https://user-images.githubusercontent.com/16904946/50252092-f9d78500-03f6-11e9-8f6c-d834b10eee95.png">
<img width="1123" alt="screenshot 2018-12-19 at 21 01 36" src="https://user-images.githubusercontent.com/16904946/50252096-ff34cf80-03f6-11e9-8367-d6cd61d92459.png">
<img width="1120" alt="screenshot 2018-12-19 at 23 27 33" src="https://user-images.githubusercontent.com/16904946/50252099-01972980-03f7-11e9-9c35-459fc700599e.png">
<img width="997" alt="screenshot 2018-12-19 at 20 33 54" src="https://user-images.githubusercontent.com/16904946/50252105-052ab080-03f7-11e9-99c0-173f8860ef24.png">

[kamyaD]

@codeplus254 Great work on this project. I tried to look at Travis and it says, Build has been terminated, could you kindly check on it?

[r-wambui]

@codeplus254 Check why the article rating model can be created test is failing.

[codeclimate[bot]]

Code Climate has analyzed commit 98379d7b and detected 1 issue on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	1

View more on Code Climate.