[m24] Sub navigation is covered by the fixed new nav on mobile screens

[wen-2018]

Description

Sub navigation is covered by the fixed new nav on mobile screens on pages that have a sub nav. Example: firefox page

Steps to reproduce

1. Set SWITCH_M24_NAVIGATION_AND_FOOTER to be true in your local env file
2. Go to http://localhost:8000/en-US/firefox/new/
3. Resize browser to mobile screen size
4. You should be able to see the subnav partially covered by the new nav

Expected result

What should happen?

The sub nav should be fully visible.

Actual result

What happened.

Environment

What browser and operating system are you using?