WIP: Refactor Authentications Controller

The piece of code with more logic for this particular controller reside in the ```AuthenticationsController#create``` method which will be the one to get more focus at first; I will list some of the things that needs to happen to get this controller in better shape and also to point out what needs to happen before merging this PR. 

- [ ] Remove temporary variables and create method instead; where applicable. 
- [ ] Introduce new service object for creation logic suggested name to follow our convention ```AuthenticationCreatorService```
- [ ] Introduce new services for most if not all private methods inside the controller; ```link_to_youtube``` should be first candidate; this looks more like control logic so probably we should discuss about where to put it. 

I've been working on this but I'm always open to suggestions.