Add Contribution Banner

[ThorntonMatthewD]

Summary

• Adds a banner to the top of the site to letting visitors know that the project is open source and where to go if they'd like to contribute.
• The banner can be "X'd out" of, and it'll remain closed for the rest of the user's session.
• The "Contribute" links in the maintaners pop-up now show as "Data Source" links.
• The maintainers pop-up now has a prominent button to guide visitors to where they can learn more about contributing.

Screenshots

_	Desktop	Mobile
Banner
Maintainer Pop-up

Testing Plan

1. Contributor Banner
   1. Verify that the banner is present at the top of the map page.
   2. Navigate to the "About" page and verify that it appears there also.
   3. Click the "X" icon on the right side of the banner.
   4. Refresh the page. The banner should not reappear.
   5. Open the page in an incognito tab. The banner shouild be back!
   6. Return back to your first window. Open your browser's dev tools and run the following in the console:

      sessionStorage.removeItem("disableHGLabsMapBanner");

   7. Refresh the page, and the banner should be back again. :partying_face:
2. Maintainer Pop-up
   1. Enable at least one feature set on the map
   2. Hover over the maintainers control icon (second one down on the right)
   3. Click on the "Click here to learn how to contribute!" button
   4. It should have taken you here in a new tab.