Activate checks each nav item in the tree of items passed
to see if the url exactly matches the current url path
and sets the .Active flag when it does
Flat creates a flatMap from the original items passed.
It marks any nav item whose url is contained within
the current request url path as being active
Level checks only the current level (does not recurse) of the nav items
passed to see if any "active" (url contains current url)
If active is found then also run the activate check on the subitems