CategoryServiceImpl.findFirstLevelCategories(): extract representation logic

[php-coder]

In the commit b7fe1da (#434) I added CategoryServiceImpl.findFirstLevelCategories() that has mix of business and representation logic. We need this conversion only because Thymeleaf lacks ability to do it (details: https://gist.github.com/php-coder/d3020e4d8d00b8c5befe755c46f06f1b).

We should extract the representation logic. This will simplify findFirstLevelCategories() method, makes testing easier and also, when Thymeleaf will support our need or if we'll change the template engine, it will be more straightforward to remove this "hack".

During refactoring we can also move FirstLevelCategoryDto to another package (for example,controller.dto).

[php-coder]

Other related commits that were linked to another issue by mistake: c78b5e5 and 7352780