Be able to qualify articles, videos and documents

- [x] Add gem
- [x] Be able to vote for articles (model)
- [x] Be able to vote for documents (model)
- [x] Be able to vote for videos (model)
- [x] Be able to vote for articles (controller)
- [x] Be able to vote for documents (controller)
- [x] Be able to vote for videos (controller)