Add rootContainer to InventoryManager object

When a user logs in, since they now have an implicit rootContainer,
track it in the InventoryManager object for use elsewhere in the app.
Traverse container tree looking for grocery lists

The getUserLists() function will traverse the container tree for the
logged in user and add all known grocery lists to the
InventoryManager['lists'] array. Each list will be an object with the
following properties:

  id: Firebase container id of the list
  name: The name of the grocery list (as specified at creation time)
  description: The grocery list description, if any