FIX: allows the selection of the default landing tab

Meta: [Where does one select the default landing tab?](http://meta.discourse.org/t/where-does-one-select-the-default-landing-tab/4819)

![Screenshot_17_03_13_00_01](https://f.cloud.github.com/assets/362783/267490/70419b26-8e8d-11e2-9e4e-8084785285b0.png)

- [x] associates the `/` route to the **first** item in the `top_menu` site setting (1) in order to mark the right item in the menu as `active` when visiting the homepage
- [x] prevents duplicate routes when the `categories` is the default landing page
- [x] removed duplicate static routes declaration

(1) my solution to retrieve the `top_menu` data from the site settings feels a bit *hacky-ish*. Unfortunately, I can't use the default `PreloadStore` API as it's a *read-only-once* structure and would delete its content.