UI minor fixes

* Reduces space between navbar menu items
* Replaces search bar with an icon
* Changes the "GEM" link (on the navbar right side)

The search now requires two click (one on the icon, the second on the search field). I would have only the click on the icon, but I'm unable to find a way to do that (GeoNode had used http://ivaynberg.github.io/select2/ to make it)

Now the platform (at least the base template) is compliant with the 1024x768 resolution.