sub menus didn't get checked for being active.

Based off the method in ```Menu.php``` I've updated ```Nav``` so it will check if sub menus are active or not.
Improved check for active child items

Updated to avoid full tree traversal.
Also noticed ```activateItems``` didn't do anything so that's been fixed.
Update CHANGELOG.md
Fixes #2805
Merge branch 'nav-active' of github.com:Alex-Code/yii2 into Alex-Code-nav-active

Conflicts:
	extensions/bootstrap/CHANGELOG.md
	extensions/bootstrap/Nav.php