implemented python-social-auth github basic login

[otivedani]

to make it works,

• store registered github application key and secrets to .env . use http://127.0.0.1:8000 as home and callback url
• start django server : python manage.py runserver
• open http://127.0.0.1:8000/social-auth/login/github/
• authorize yourself using github account
• done.

todo:

• [ ] reconsider auth definitions : settings_base.py or settings.py

[otivedani]

moving to #9 (wagtail base)