Navigation DropDown fix for mobile devices
Fix approach is taken from Twitter Bootstrap 3.0

Twitter Bootstrap bug reports:
https://github.com/twbs/bootstrap/issues/4550
https://github.com/twbs/bootstrap/pull/6488
Navigation DropDown fix for mobile devices
Fix approach is taken from Twitter Bootstrap 3.0

Twitter Bootstrap bug reports:
https://github.com/twbs/bootstrap/issues/4550
https://github.com/twbs/bootstrap/pull/6488