Avoid loading styles twice

Change-Id: I8a394cac1e742d3c522f741ebb0f41e3f1ef8bac
Several improvements to the navbar

* split off navmenu into its own class
* page tools: allow hiding link to current namespace
* page tools: don't display if there are no tools (e.g. Special Pages)
* improve navhead.xml (flatten nav menu, hide link to current namespace)
* fix z-index of navbars

Change-Id: Iba41064af7fe36946143841d55bde2860ac91cb4