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 vulnerability link in 'Share' now points to ```/vulnerability/list/```
* Vulnerability header menu background is now flat gray 

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:
![screenshot from 2014-11-03 16 59 48](https://cloud.githubusercontent.com/assets/1818657/4885172/7cf5a270-6372-11e4-8505-4f61ff266f0f.png)