Add file download icon to de-risking guide primary nav

[nateborr]

Changes proposed in this pull request:

Display an icon next to download links in the primary navigation. Currently this is used only in the de-risking guide.

Resolves #704.

Approach

When the page is at least 64em wide and the primary nav is shown in the page header, conditionally set a 1-pixel top margin on the icon in order to optically center it. This isn't necessary in narrower screens, when the primary nav moves to the Menu drop-down. This is implemented within the existing media rule for the extended header styles.

Testing

Manually test the primary nav in this PR's build.

• Visit any page in the de-risking guide.
• Confirm that the "Download" link shows the USWDS file download icon to its right when the viewport is...
  1. wide enough that the primary nav is shown at the top of the page
  2. narrow enough that the primary nav is shown within the "Menu" dropdown
• Check that all primary links don't include icons, appear correctly, and work as expected.

security considerations

None. This change uses the existing framework.

[melchoyce]

Is there any way to push the icon down 1px so it looks a little more optically centered? Otherwise, LGTM 👍

[nateborr]

I'll rework this to adjust the download icon positioning and, as part of that, make the icon handling more specific to the download link.

[nateborr]

@melchoyce I've updated the icon positioning per your review. It now has a 1-pixel top margin when the primary nav is displayed in the header.

[melchoyce]

LGTM!

[peterrowland]

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