add social media sharing buttons to the sharing div

[fiveham]

Empowering users to share the map on social media easily would really help make this tool more useful.

One thing is unclear at the moment: How do the trademark and copyright rights of the many IP owners involved interact with the license I've put this project under? Is there a contradiction? Is it totally okay? Are there just some extra hoops to jump through before doing that?

[fiveham]

Side issue: before adding social media sharing links (esp. Facebook), should add the rest of the Open Graph sort of stuff to the head. Apparently that's important for social media social graph stuff.

[fiveham]

Will not fix / done.

Would have been wise for 2018, but didn't get around to that sort of thing until the 2020 NC primary map.