Update navigation design for admin settings

Created by: 5h1rU

Overview

This PR implements the updated navigation design for the admin setting area.

Screenshot / Video

BEFORE

AFTER

https://user-images.githubusercontent.com/989826/109346655-fb573000-783f-11eb-9e69-8f858783f860.mov

Progress

• Documentation text/screenshots updated (if relevant)
• Approved by a frontend engineer (if touching frontend code)
• Approved by a designer (if it touches the UI)

Closes https://github.com/sourcegraph/sourcegraph/issues/13375