This change swaps out `Category.findById` with `Category.findBySlugPathWithID`. It ends up reducing the necessary code and handles the scenario of using a link without a numeric ID (e.g. `/tags/c/some-category/some-tag` instead of `/tags/c/some-category/4/some-tag`).

This also simplifies the routing logic in line with what was discussed in #8. There's no need to create an `isCategoryTopicList` variable, especially now that the code is a little less busy.