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.
Not sure if we should depend on system (is it even possible on heroku?) ability to set array as an environment variable. Also scope isn't secret, and `_local.env.js` is excluded from git, so this change would make app migration more difficult (one cannot obtain previously set scope from "developer console"). 

Regarding Google scope, seems like there are [some changes](https://developers.google.com/+/api/auth-migration) on a horizon and scope should be changed to: `scope: ['profile', 'email']`.