Navigation: Gradient on Scrollable Navigation is wrong width > 400px

[tochwill]

Describe the bug
The gradient should be gradient 6em above 400px page width - it currently remains at 3em.

To Reproduce
Steps to reproduce the behaviour:

1. Go to any page containing the navigation
2. See error

Expected behaviour
The gradient should be gradient 6em above 400px page width. Use the correct constants from Psammead to achieve this.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Testing notes

• This bug fix is expected to need manual testing.

Additional context
Add any other context about the problem here.