GTNPORTAL-2814 - Fixing several CSS/JS issues

Hi, 

Regarding to last e-mail response from Gabriel, I'm fixing several issues in the Navigation portlet:
* Media query for vertical design is now cca 600px.
* Menus for big screen comply to the original design (=are horizontal).
* Sub-menus are CSS-ready for two levels (indentation of sub-menus in vertical design works for only two levels). More levels do work, but are considered a bad design for drop-down menus.
* Menus are now responding to the window (resize or zoom) changes - They won't go outside the screen if it's possible.
* Currently opened node has now black text colour providing visual hint.

I hope we're clear on the design now so I can just fix bugs in next PRs, because I need to move on to another task.