MDL-45893 user_menu

Changes so far:
 - patched bootstrap JS to allow for multiple navbar items (code is
   backwards-compatible with existing usage)
 - separated navbar layout file out into its own file under /includes
 - initial renderable and renderer for navbar dropdown collections which
   totally works
 - ported across element-inclusion helper from fred's easy theme
 - actually render a user menu
 - show a page_heading_menu if present

To do:
 - style the user menu, custom menu, and pageheader menu on desktop
 - style the menus on mobile / narrow screens
 - figure out what page_heading_menus are actually used for