Add tags to wiki topics and articles (#166)

* Make `WikiTopic` a `CRUDMixin` subclass

* Make `WikiArticle` a `CRUDMixin` subclass

* Add "description" field to `WikiTopic` and `WikiArticle` DB models and the routes using them

* Make some wiki routes more future-proof

* Add a route to update topic

* Fix `WikiTopic` repr

* Add `WikiTag` DB model that allows to add tags to topics and articles

* Slugify `WikiTag.name` at creation

* Nits

* Simplify `WikiTag` model

* More nits