Frontend/python social auth

This PR implementation social login with [python-social-auth](https://github.com/omab/python-social-auth)

Configuration
------------------
```python
SECRET_KEY = 'AASDFASFADSFDF'

SOCIAL_AUTH_TWITTER_KEY = 'hva0jBn4bwC5JCLQCxLWM3RN0'
SOCIAL_AUTH_TWITTER_SECRET = 'ykGRcLOlUETudGFpWWpoGNnmP6qerGUTB7cOQaTZbI9ucQp1I5'
```

Usage
---------
connect to [http://127.0.0.1:5000/user/](http://127.0.0.1:5000/user/) to try login.

Known Issue (but I have no idea.)
----------------------------------------------
1. test is missing.
@dahlia I found its usage.

https://github.com/omab/python-social-auth/blob/13fa6c2dc1483b20c3ae574cfd9640ad563f6aee/social/utils.py#L85