[WIP] Add topics

This work started on the meetup along with @kamal-hothi.

Is this going to the expected way? Feedback, please! :smile: 

* [x] Add topics as a `has_and_belongs_to_many` to questions. A question can be part of multiple topics;
* [x] Add shoulda-matchers to easier specs to validation, relations;
* [x] Add missing specs to validations and relations to Question;
* [x] Add basic administration to Topic on active admin;
* [x] Fix `db/seeds.rb` to add a "General" topic
* [ ] Relate questions and topics on active admin
* [ ] Modify questions#index to show topics
* [ ] Add a topics#index view