Fixed sticky MR tabs positioning when sidebar is pinned

## What does this MR do?

The sticky MR tabs where positioned underneath the pinned sidebar. This fixes that by accounting for the size of the pinned nav.

## Screenshots (if relevant)

![Screen_Shot_2016-10-28_at_09.37.18](/uploads/0d23f2bb0e02d698c012c22c8653afd7/Screen_Shot_2016-10-28_at_09.37.18.png)

## What are the relevant issue numbers?

Closes #23926

See merge request !7167