[WiP] Feature/blog categories

Add categories to the blog

- [x] Add category entities
- [x] Load category with article
- [x] Create route to load articles based on category
- [x] Create admin pages to CRUD categories
- [x] Create relation to set category_id to NULL when category is deleted
- [x] Update article form to have a select for the available categories
- [x] Attach NullValue strategy from Soflomo\Common to article's category_id
- [x] Update route for controller with identifier or slug, not the name