Scrolls active tab into middle of nav on mobile

## What does this MR do?

On mobile (or smaller screens) it scrolls the currently active tab into the middle of the nav.

## Screenshots (if relevant)

![Screen_Shot_2016-09-15_at_09.38.55](/uploads/4170271b53b9509a385fa7844fb3dc78/Screen_Shot_2016-09-15_at_09.38.55.png)

See merge request !6363