Chore/rails best practices

## Summary
Using rails best practices gem and eliminate all warnings

* Add decent_exposure gem
* Refactor controllers:
  - Remove instance variables from views
* Simplify render from controllers
* In the end: No warning found. Cool!
also you can expose `badge` and `badges`
`expose(:badge, attributes: :badge_params)`
`expose(:badges) { Badges.all }`, well... this is not really necesary XD