content/static: fix header styling problems

+ On Firefox, the menu items were not vertically aligned. Fixed by
  ensuring the nav element is display: flex.
+ Don’t show unwanted border radius, margin, and padding
  on mobile devices and desktop Safari.
+ Adjust horizontal padding on smaller viewports.
+ Underline links when they’re focused.

Updates golang/go#9936

Change-Id: I2fb65a559a0498660b6d863a7fe29c8347cdc3ce
Reviewed-on: https://go-review.googlesource.com/c/website/+/178437
Reviewed-by: Katie Hockman <[email protected]>

Author: andybons