base: navigation menu buttons cleanup

* Removes inline javascript and css.

* Adds `navbar-btn` class to login and register buttons.

Signed-off-by: Harris Tzovanakis <me@drjova.com>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
communities: featured community addition

* Adds featured community.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>