Fix mobile navigation scrolling bug

Fixed a small bug where the mobile navigation was collapsing so items were disappearing off the side. Removes the requirement to have a magic-number for the offset which needed updating each time the navigation changed.

**Before**
![mobile-nav-before mov](https://cloud.githubusercontent.com/assets/123386/5487703/c9171852-86ac-11e4-9e97-9b4d066b5340.gif)

**After**
![mobile-nav-after mov](https://cloud.githubusercontent.com/assets/123386/5487704/cc818d56-86ac-11e4-9f4c-8ffaf174290e.gif)

Not 100% happy with this as it's a magic-number. The height of the navigation needs to be the same height as the navigation toggle button, which doesn't have a fixed height, just happens to be `36px`.