added hover state to top nav links

## What does this MR do?

Adds hover state to top nav links.

## Are there points in the code the reviewer needs to double check?

Ney.

## Why was this MR needed?

UX.

## What are the relevant issue numbers?

Closes #18301.

## Screenshots (if relevant)

![Screen_Shot_2016-06-07_at_23.23.45](/uploads/dac8ed99eca99fbcccbd98028e947ee1/Screen_Shot_2016-06-07_at_23.23.45.png)

See merge request !4521