Update gray block under subnav to match other pages

## What does this MR do?
Makes Compare sub nav full width; replaces gray block with white one to match rest of pages

## What are the relevant issue numbers?
Closes #18934 

## Screenshots (if relevant)
![Screen_Shot_2016-06-21_at_1.14.46_PM](/uploads/43377efe7816b854f48966bcbd96a1cc/Screen_Shot_2016-06-21_at_1.14.46_PM.png)

cc @dzaporozhets 

See merge request !4838