Utilize categories to control advert automation

Places can have a set of categories, which are used to control the display candidates of a place. The main idea behind categories is that we sometimes want to display adverts at strategically good places. E.g. we might have a couple of posts about healthy food and the advert about a new kind of diet should be displayed there. This is achieved by inserting places at these posts and tag them with a category nutrition. Then every advert with the same category is displayed there more likely. This also holds for adverts which are created in the future and tagged with the category nutrition.