New dependecy manager, authentication and code formatter

Improvements:

• Upgrade dependencies
• Go back to pip + requirements as dependency manager
• Replace allauth + django-rest-auth with ckl-rest-auth as authentication lib
• Add custom user definition as default
• Replace flake8 by black as code formatter

New folder structure + Django 2.0

Merge pull request #29 from CheesecakeLabs/bernardo/boilerplate-v2

Update project folders structure and dependencies version