MDL-45893 navigation: create a user menu
Changelist
 - renderable based on custom_menu
 - support for MNet users and role changing back
 - user menu rendering on Base-based themes
 - guest users
 - patched bootstrap JS to allow for multiple navbar items (code is
   backwards-compatible with existing usage)
 - user menu rendering on Bootstrapbase-based themes
 - fix for custom_menu dropdown toggles with subnodes
 - responsive user and custom menus
 - page header menu
 - RTL