Rework NavbarHorizontal for responsiveness

* extensive reworks of NavbarHorizontal.php and .less
* right-aligned subcomponents of navbars (searchbar and personal tools) are now
  wrapped and floated right en bloc to keep the element order as specified in
  the layout file
* new 'position' attribute for sub-components of NavbarHorizontal component
* PersonalTools and SearchBar are not floated right by default anymore
* 'jquery-sticky' module is now registered in SetupAfterCache hook
* new Less variables for link colors: @navbar-user-not-loggedin,
  @navbar-newtalk-not-available

Change-Id: I14e9243cd6d7ceff6c83c9a8208b20c26b7d1555