Create Members module with member list page

[tadhgboyle]

• Allows modules to register custom member lists to display on the member list page
• Allows site admins to toggle display of each member list
• Allows modules to add some metadata about each member list user to display underneath their name

TODO:

• make nav item a dropdown:
  • links for active member lists (under main header)
  • header for "groups"
  • all active groups under this header

outdated