Add global navigation menu

As per design. Replaces the dynamic render_main_menu method with
‘static’ HTML due to constraints with the render_main_menu and
active_link_to methods.
Remove irrelevant comment
Add correct locale keys
Add menu link component for all links and sublinks on the main page
Refactor the entire main menu, replacing and add the menu link component
Added active state for dropdown button on global navigation
Add I18n locale for the sign out text
Add extra line to users locale
Remove extra line from menu link cms component
Only shows the navigation when site is persisted.
Merge pull request #78 from moneyadviceservice/global-navigation

Global navigation