Add Django backend to store and compare issues

[La0]

Django web application to store & compare code-review issues:

• Integrated to Taskcluster CI / CD
• Running on Heroku: https://code-review-backend-dev.herokuapp.com/
• Load existing issues by reading the bot's report.json files
• Rest API to list, retrieve and create the Revision, Diff & Issue models
• Unit tests for the API
• Automatic Django admin to browse the data
• Anonymous users can read data, logged in users can create data

[La0]

I split that PR in #161 + #162. Closing this one.