Remove active navigation item underline overhanging edge of navbar

[robbieaverill]

Version: 3.x

The underline currently overhands the navbar. New designs indicate it should be inside its container instead.

Original:

New designs: https://invis.io/AWO7CJSK426#/331465174_Wa-tea_Plus_Landing_Page

cc @silverstripeux

[clarkepaul]

This got pushed out of the backlog due to not enough time. We have a bunch more stories which are enhancements which we will add. I would say that unless you have the time we just raise it as an enhancement—which you have done :).