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.
I think they should stay where they are now. One of the reasons would be: that's exactly where I would be looking for them :).

Also, putting them in `config/environment/index.js` would prevent us from using your trick with just passing reference to them as a first param in `{Facebook,Google,Twitter}Strategy`.

So just let them be where they are now.