feat(oauth): remove code according to user prompts

Add one global oauth filter that is set to true when we have at lease one oAuth strategy to simplify some conditions.

Remove all code that is not necessary depending on what is taken at the initialization.

Applied in style files, login and signup controllers / views, user model and env config file.
In order to make some refactoring in `server/auth/`, could we add:
```
FACEBOOK_SCOPE: ['email', 'user_about_me'],
GOOGLE_SCOPE: [
      'https://www.googleapis.com/auth/userinfo.profile',
      'https://www.googleapis.com/auth/userinfo.email'
    ]
```
(For Twitter, there is no scope)