fix: secondary nav offset, dayjs import in utils/filters.js

[stephiescastle]

Checklist

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Description

Supports https://github.com/nasa-jpl/www/issues/244

Instructions to test

• difficult to test without using the www frontend app (requires publishing the npm package for general testing).
• to test locally, you can follow these instructions to reference your local copy of explorer-1. Then:

1. make backend
2. make frontend
3. make a content page and check "override secondary navigation"
4. create child pages and check "show in menus"
5. view the first content page you made on the frontend. You should see the secondary navigation stick to the top when scrolling. When scrolling back up, the main nav should appear, and the secondary nav should be offset just enough.

Tested in the following environments/browsers:

Operating System

• macOS
• iOS
• iPadOS
• Windows

Browser

• Chrome
• Firefox ESR
• Firefox
• Safari
• Edge