Lazy load Groups/Users menu

This fixes the slow loading of the main webclient pages when you are an Admin with lots of groups.
The drop-down groups/users menu is only loaded on demand, not when the page is loaded.
However, groups that you are a member of are still loaded with the main page, since they are needed for 'Move to Group' right-click menu, and on Search page.

To test:
 - Check that Groups/Users menu works as before on various pages (home, history, tags, public).
 - After switching Group/User, refresh of *same* page.
 - Check that 'Move to Group' still lists groups OK and works.
 - Check that search page still lists Groups and Colleagues correctly and works.